webworka

Just as an remark: This misconfiguration also has the affect that you can no longer login to the backend because the action of the login-form is also replaced with the CDN-URL

I found the problem :-/

The reason was a misconfiguration in the advanced section of the CDN settings. The field Custom file list: had one line with the domain name of the page in it. This lead to a strange replacement in the inside js and also the action of form elements.

I removed the line and now everything works fine.