upgrading apache2 from php5.6 to php7.3 gives segmentation fault on mod_ssl

Problem description

I have an server that is running debian 8.11, with an apache2 server currently using PHP 5.6.

I want to upgrade to PHP 7.3 and have installed PHP 7.3 with apt ( php7.3 php7.3-fpm php7.3-mysql php7.3-curl php7.3-json php7.3-gd php7.3-intl php7.3-mbstring php7.3-xml php7.3-zip php7.3-exif php7.3-apcu).

I know that I should be able to change the PHP version with apache by running a2dismod php5 and a2enmod php7.3. Though, when I then do a configuration test with sudo apache2ctl configtest, I get a segmentation fault:

$ sudo apache2ctl configtest
Syntax OK
Segmentation fault
Action 'configtest' failed.

After some trial and error the segmentation fault is gone when I disable the SSL module (sudo a2dismod ssl), but I want to be able to serve the website over HTTPS, just as I currently do under PHP 5.6.

Question

I can’t find out what I must change in my SSL configuration with the upgrade from PHP 5 to PHP 7. Can someone help?

Files you might want to see

ssl.load

# Depends: setenvif mime socache_shmcb
LoadModule ssl_module /usr/lib/apache2/modules/mod_ssl.so

ssl.conf

<IfModule mod_ssl.c>

        # Pseudo Random Number Generator (PRNG):
        # Configure one or more sources to seed the PRNG of the SSL library.
        # The seed data should be of good random quality.
        # WARNING! On some platforms /dev/random blocks if not enough entropy
        # is available. This means you then cannot use the /dev/random device
        # because it would lead to very long connection times (as long as
        # it requires to make more entropy available). But usually those
        # platforms additionally provide a /dev/urandom device which doesn't
        # block. So, if available, use this one instead. Read the mod_ssl User
        # Manual for more details.
        #
        SSLRandomSeed startup builtin
        SSLRandomSeed startup file:/dev/urandom 512
        SSLRandomSeed connect builtin
        SSLRandomSeed connect file:/dev/urandom 512

        ##
        ##  SSL Global Context
        ##
        ##  All SSL configuration in this context applies both to
        ##  the main server and all SSL-enabled virtual hosts.
        ##

        #
        #   Some MIME-types for downloading Certificates and CRLs
        #
        AddType application/x-x509-ca-cert .crt
        AddType application/x-pkcs7-crl .crl

        #   Pass Phrase Dialog:
        #   Configure the pass phrase gathering process.
        #   The filtering dialog program (`builtin' is a internal
        #   terminal dialog) has to provide the pass phrase on stdout.
        SSLPassPhraseDialog  exec:/usr/share/apache2/ask-for-passphrase

        #   Inter-Process Session Cache:
        #   Configure the SSL Session Cache: First the mechanism
        #   to use and second the expiring timeout (in seconds).
        #   (The mechanism dbm has known memory leaks and should not be used).
        #SSLSessionCache                 dbm:${APACHE_RUN_DIR}/ssl_scache
        SSLSessionCache         shmcb:${APACHE_RUN_DIR}/ssl_scache(512000)
        SSLSessionCacheTimeout  300

        #   Semaphore:
        #   Configure the path to the mutual exclusion semaphore the
        #   SSL engine uses internally for inter-process synchronization.
        #   (Disabled by default, the global Mutex directive consolidates by default
        #   this)
        #Mutex file:${APACHE_LOCK_DIR}/ssl_mutex ssl-cache

        #   SSL Cipher Suite:
        #   List the ciphers that the client is permitted to negotiate. See the
        #   ciphers(1) man page from the openssl package for list of all available
        #   options.
        #   Enable only secure ciphers:
        SSLCipherSuite HIGH:!aNULL

        # SSL server cipher order preference:
        # Use server priorities for cipher algorithm choice.
        # Clients may prefer lower grade encryption.  You should enable this
        # option if you want to enforce stronger encryption, and can afford
        # the CPU cost, and did not override SSLCipherSuite in a way that puts
        # insecure ciphers first.
        # Default: Off
        #SSLHonorCipherOrder on

        #   The protocols to enable.
        #   Available values: all, SSLv3, TLSv1, TLSv1.1, TLSv1.2
        #   SSL v2  is no longer supported
        SSLProtocol all -SSLv3

        #   Allow insecure renegotiation with clients which do not yet support the
        #   secure renegotiation protocol. Default: Off
        #SSLInsecureRenegotiation on

        #   Whether to forbid non-SNI clients to access name based virtual hosts.
        #   Default: Off
        #SSLStrictSNIVHostCheck On

</IfModule>

# vim: syntax=apache ts=4 sw=4 sts=4 sr noet

apache2.conf

# This is the main Apache server configuration file.  It contains the
# configuration directives that give the server its instructions.
# See http://httpd.apache.org/docs/2.4/ for detailed information about
# the directives and /usr/share/doc/apache2/README.Debian about Debian specific
# hints.
#
#
# Summary of how the Apache 2 configuration works in Debian:
# The Apache 2 web server configuration in Debian is quite different to
# upstream's suggested way to configure the web server. This is because Debian's
# default Apache2 installation attempts to make adding and removing modules,
# virtual hosts, and extra configuration directives as flexible as possible, in
# order to make automating the changes and administering the server as easy as
# possible.

# It is split into several files forming the configuration hierarchy outlined
# below, all located in the /etc/apache2/ directory:
#
#       /etc/apache2/
#       |-- apache2.conf
#       |       `--  ports.conf
#       |-- mods-enabled
#       |       |-- *.load
#       |       `-- *.conf
#       |-- conf-enabled
#       |       `-- *.conf
#       `-- sites-enabled
#               `-- *.conf
#
#
# * apache2.conf is the main configuration file (this file). It puts the pieces
#   together by including all remaining configuration files when starting up the
#   web server.
#
# * ports.conf is always included from the main configuration file. It is
#   supposed to determine listening ports for incoming connections which can be
#   customized anytime.
#
# * Configuration files in the mods-enabled/, conf-enabled/ and sites-enabled/
#   directories contain particular configuration snippets which manage modules,
#   global configuration fragments, or virtual host configurations,
#   respectively.
#
#   They are activated by symlinking available configuration files from their
#   respective *-available/ counterparts. These should be managed by using our
#   helpers a2enmod/a2dismod, a2ensite/a2dissite and a2enconf/a2disconf. See
#   their respective man pages for detailed information.
#
# * The binary is called apache2. Due to the use of environment variables, in
#   the default configuration, apache2 needs to be started/stopped with
#   /etc/init.d/apache2 or apache2ctl. Calling /usr/bin/apache2 directly will not
#   work with the default configuration.


# Global configuration
#

#
# ServerRoot: The top of the directory tree under which the server's
# configuration, error, and log files are kept.
#
# NOTE!  If you intend to place this on an NFS (or otherwise network)
# mounted filesystem then please read the Mutex documentation (available
# at <URL:http://httpd.apache.org/docs/2.4/mod/core.html#mutex>);
# you will save yourself a lot of trouble.
#
# Do NOT add a slash at the end of the directory path.
#
#ServerRoot "/etc/apache2"

#
# The accept serialization lock file MUST BE STORED ON A LOCAL DISK.
#
Mutex file:${APACHE_LOCK_DIR} default

#
# PidFile: The file in which the server should record its process
# identification number when it starts.
# This needs to be set in /etc/apache2/envvars
#
PidFile ${APACHE_PID_FILE}

#
# Timeout: The number of seconds before receives and sends time out.
#
Timeout 300

#
# KeepAlive: Whether or not to allow persistent connections (more than
# one request per connection). Set to "Off" to deactivate.
#
KeepAlive On

#
# MaxKeepAliveRequests: The maximum number of requests to allow
# during a persistent connection. Set to 0 to allow an unlimited amount.
# We recommend you leave this number high, for maximum performance.
#
MaxKeepAliveRequests 100

#
# KeepAliveTimeout: Number of seconds to wait for the next request from the
# same client on the same connection.
#
KeepAliveTimeout 5


# These need to be set in /etc/apache2/envvars
User ${APACHE_RUN_USER}
Group ${APACHE_RUN_GROUP}

#
# HostnameLookups: Log the names of clients or just their IP addresses
# e.g., www.apache.org (on) or 204.62.129.132 (off).
# The default is off because it'd be overall better for the net if people
# had to knowingly turn this feature on, since enabling it means that
# each client request will result in AT LEAST one lookup request to the
# nameserver.
#
HostnameLookups Off

# ErrorLog: The location of the error log file.
# If you do not specify an ErrorLog directive within a <VirtualHost>
# container, error messages relating to that virtual host will be
# logged here.  If you *do* define an error logfile for a <VirtualHost>
# container, that host's errors will be logged there and not here.
#
ErrorLog ${APACHE_LOG_DIR}/error.log

#
# LogLevel: Control the severity of messages logged to the error_log.
# Available values: trace8, ..., trace1, debug, info, notice, warn,
# error, crit, alert, emerg.
# It is also possible to configure the log level for particular modules, e.g.
# "LogLevel info ssl:warn"
#
LogLevel warn

# Include module configuration:
IncludeOptional mods-enabled/*.load
IncludeOptional mods-enabled/*.conf

# Include list of ports to listen on
Include ports.conf


# Sets the default security model of the Apache2 HTTPD server. It does
# not allow access to the root filesystem outside of /usr/share and /var/www.
# The former is used by web applications packaged in Debian,
# the latter may be used for local directories served by the web server. If
# your system is serving content from a sub-directory in /srv you must allow
# access here, or in any related virtual host.
<Directory />
        Options FollowSymLinks
        AllowOverride None
        Require all denied
</Directory>

#<Directory /usr/share>
#       AllowOverride None
#       Require all granted
#</Directory>

<Directory /var/www/>
        Options FollowSymLinks
        AllowOverride None
        Require all granted
</Directory>

#<Directory /srv/>
#       Options Indexes FollowSymLinks
#       AllowOverride None
#       Require all granted
#</Directory>




# AccessFileName: The name of the file to look for in each directory
# for additional configuration directives.  See also the AllowOverride
# directive.
#
AccessFileName .htaccess

#
# The following lines prevent .htaccess and .htpasswd files from being
# viewed by Web clients.
#
<FilesMatch "^.ht">
        Require all denied
</FilesMatch>


#
# The following directives define some format nicknames for use with
# a CustomLog directive.
#
# These deviate from the Common Log Format definitions in that they use %O
# (the actual bytes sent including headers) instead of %b (the size of the
# requested file), because the latter makes it impossible to detect partial
# requests.
#
# Note that the use of %{X-Forwarded-For}i instead of %h is not recommended.
# Use mod_remoteip instead.
#
LogFormat "%v:%p %h %l %u %t "%r" %>s %O "%{Referer}i" "%{User-Agent}i"" vhost_combined
LogFormat "%h %l %u %t "%r" %>s %O "%{Referer}i" "%{User-Agent}i"" combined
LogFormat "%h %l %u %t "%r" %>s %O" common
LogFormat "%{Referer}i -> %U" referer
LogFormat "%{User-agent}i" agent

# Include of directories ignores editors' and dpkg's backup files,
# see README.Debian for details.

# Include generic snippets of statements
IncludeOptional conf-enabled/*.conf

# Include the virtual host configurations:
IncludeOptional sites-enabled/*.conf

# vim: syntax=apache ts=4 sw=4 sts=4 sr noet

LoadModule proxy_module /usr/lib/apache2/modules/mod_proxy.so
LoadModule proxy_wstunnel_module /usr/lib/apache2/modules/mod_proxy_wstunnel.so

#ProxyRequests off
#SSLProxyEngine on
#SSLProxyVerify none
#SSLProxyCheckPeerCN off
#SSLProxyCheckPeerName off
ProxyPass /wss2/ ws://mywebaddress.com:4433