@db_rab,
The deactivation of security plugin doesn't help because they have already configured and hardened the .htaccess file.
Please try to deactivate the security and cache plugins then, backup your current .htaccess fie and change it to the initial WordPress one, just remove all and keep this part:
# 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
Save it, and check it again. Make sure the plugins are deactivated.
Don't forget to backup your current .htaccess file to be able restore it after the testing.