@organu @ringostar77 we are also using Plesk. I checked our .htaccess files and they look good for WordPress. I also disabled every Plugin and the question mark is still getting prefixed to our URL’s.
Our permalink settings (wp-admin/options-permalink.php) are set to use Post name. If I change this to Plain the website works as the question mark is needed anyways for the URL ?p=123 or ?=page_id=123 but with Plain you do not use question marks.
In Plesk when I go to the section where the domain is, under “Apache & nginx Settings”, I have the following settings:
MIME types: default
Handlers: Default
Deny access to the site: Default
Index files: Default
Expires: Default
Additional Headers: Default
Restrict the ability to follow symbolic links: not enabled
Proxy mode: Enabled
Smart static file processing: Enabled
Serve static files directly by nginx: not enabled
Enable nginx caching: not enabled