Hi,
Just found the issue myself. It was not the /wp-admin/.htaccess , it was the root folder .htaccess, where I had the additional 6G Firewall code in Custom Code #14 which blocked the request. In detail it were the rules:
RedirectMatch 403 (:)
and
RedirectMatch 403 (?i)(https?):/
…in root folder .htaccess, which were responsible for the issue.
So, the Event Code: WPADMIN-SBR was misleading in this case. But solved 🙂