WordPress wp-admin error 403 files in wp-admin are accessable – Plesk

I have a blog running and everything seems to be fine.

But I cannot access

/wp-admin/ ==> 403

it is only that directory, which is not accessable…

I can easily access /wp-admin/edit.php and every other file within this directory (also /wp-admin/index.php is working…)

directory permission is set to 755

htaccess from blog root:

# BEGIN WordPress
# Die Anweisungen (Zeilen) zwischen „BEGIN WordPress“ und „END WordPress“ sind
# dynamisch generiert und sollten nur über WordPress-Filter geändert werden.
# Alle Änderungen an den Anweisungen zwischen diesen Markierungen werden überschrieben.
<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

Website is running on a root server with plesk installed.

Does anyone have an idea how I can fix this?

thank you so much
cheers
D