WordPress: 403 error when trying to access a php page in my plugin's directory

It works fine on my local server, but when I try to do it on the live server, it gives me 403 error.

Basically I have a plugin made, which has a settings page in the WordPress dashboard. In that settings page, there is a form with the action attribute leading to a different php page, that is located in my plugin’s directory. On localhost, when I submit the form, the php page opens and the script executes. But when I try to submit the form on the live server, I get a 403 error when the form redirects to the php page.

I already checked file permissions using sftp and permissions are correct.

This is the htaccess file of the live server:

# Begin GzipofBreezeWPCache
<IfModule mod_env.c>
SetEnv BREEZE_GZIP_ON 1
</IfModule>
<IfModule mod_deflate.c>
AddType x-font/woff .woff
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE image/svg+xml
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE text/xml
AddOutputFilterByType DEFLATE text/css
AddOutputFilterByType DEFLATE text/vtt
AddOutputFilterByType DEFLATE text/x-component
AddOutputFilterByType DEFLATE text/javascript
AddOutputFilterByType DEFLATE application/js
AddOutputFilterByType DEFLATE application/x-httpd-php
AddOutputFilterByType DEFLATE application/x-httpd-fastphp
AddOutputFilterByType DEFLATE application/atom+xml
AddOutputFilterByType DEFLATE application/json
AddOutputFilterByType DEFLATE application/ld+json
AddOutputFilterByType DEFLATE application/x-web-app-manifest+json
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/javascript
AddOutputFilterByType DEFLATE application/x-javascript
AddOutputFilterByType DEFLATE application/x-font-ttf
AddOutputFilterByType DEFLATE application/vnd.ms-fontobject
AddOutputFilterByType DEFLATE font/opentype
AddOutputFilterByType DEFLATE font/ttf
AddOutputFilterByType DEFLATE font/eot font/otf
AddOutputFilterByType DEFLATE font/otf
AddOutputFilterByType DEFLATE font/woff
AddOutputFilterByType DEFLATE application/x-font-woff
AddOutputFilterByType DEFLATE application/font-woff2
AddOutputFilterByType DEFLATE image/x-icon
</IfModule>
# End GzipofBreezeWPCache
#Expires headers configuration added by BREEZE WP CACHE plugin
<IfModule mod_env.c>
 SetEnv BREEZE_BROWSER_CACHE_ON 1
</IfModule>
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault "access plus 1 month"
# Assets
ExpiresByType text/css "access plus 1 month"
ExpiresByType application/javascript "access plus 1 month"
ExpiresByType application/x-javascript "access plus 1 month"
ExpiresByType text/javascript "access plus 1 month"
# Media assets 
ExpiresByType audio/ogg "access plus 1 year"
ExpiresByType image/bmp "access plus 1 year"
ExpiresByType image/gif "access plus 1 year"
ExpiresByType image/jpeg "access plus 1 year"
ExpiresByType image/png "access plus 1 year"
ExpiresByType image/svg+xml "access plus 1 year"
ExpiresByType image/webp "access plus 1 year"
ExpiresByType video/mp4 "access plus 1 year"
ExpiresByType video/ogg "access plus 1 year"
ExpiresByType video/webm "access plus 1 year"
# Font assets 
ExpiresByType application/vnd.ms-fontobject "access plus 1 year"
ExpiresByType font/eot "access plus 1 year"
ExpiresByType font/opentype "access plus 1 year"
ExpiresByType application/x-font-ttf "access plus 1 year"
ExpiresByType application/font-woff "access plus 1 year"
ExpiresByType application/x-font-woff "access plus 1 year"
ExpiresByType font/woff "access plus 1 year"
ExpiresByType application/font-woff2 "access plus 1 year"
# Data interchange
ExpiresByType application/xml "access plus 0 seconds"
ExpiresByType application/json "access plus 0 seconds"
ExpiresByType application/ld+json "access plus 0 seconds"
ExpiresByType application/schema+json "access plus 0 seconds"
ExpiresByType application/vnd.geo+json "access plus 0 seconds"
ExpiresByType text/xml "access plus 0 seconds"
ExpiresByType application/rss+xml "access plus 1 hour"
ExpiresByType application/rdf+xml "access plus 1 hour"
ExpiresByType application/atom+xml "access plus 1 hour"
# Manifest files
ExpiresByType application/manifest+json "access plus 1 week"
ExpiresByType application/x-web-app-manifest+json "access plus 0 seconds"
ExpiresByType text/cache-manifest  "access plus 0 seconds"
# Favicon
ExpiresByType image/vnd.microsoft.icon "access plus 1 week"
ExpiresByType image/x-icon "access plus 1 week"
# HTML no caching
ExpiresByType text/html "access plus 0 seconds"
# Other
ExpiresByType application/xhtml-xml "access plus 1 month"
ExpiresByType application/pdf "access plus 1 month"
ExpiresByType application/x-shockwave-flash "access plus 1 month"
ExpiresByType text/x-cross-domain-policy "access plus 1 week"
 </IfModule>
#End of expires headers configuration
# BEGIN WordPress
# The directives (lines) between "BEGIN WordPress" and "END WordPress" are
# dynamically generated, and should only be modified via WordPress filters.
# Any changes to the directives between these markers will be overwritten.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress