I have an own server, and suddently one of my accounts (domains) stops making logfiles... Does any server-guru know what's wrong and how to fix it?

thanks :)

You are talking about the web server logs? If so then XOOPS would not affect that in any way.

Check the server config and verify where the logs are supposed to be including the error log. If the location is in the same directory as other logs (and the config is correct) then the most common reason is a failed log rotation. You might still be writing to the rotated log file or into /dev/null. A web server restart should restart the log in the right place.