Now in the same directory I found the file "comment_service.php"
It had the following two lines:
//$this->configs[self::CONFIG_ALLOWED_TAGS] = array('a', 'b', 'i', 'span', 'u', 'strong', 'br');
//$this->configs[self::CONFIG_ALLOWED_ATTRS] = array('style', 'href');
Thinking I can allow HTML tags in comments when uncommenting those two lines, I did exeactly that. But after it I could no longer comment on anything.
So I reverted everything and commented out the two lines again. The file is now exactly as it was before - but I still CANNOT comment anything on my site :(
I tried enabling/disenabling de DEV mode but no result.
I am very sorry for bothering you guys due to my adventurious editing of core-files... :( But any idea out there of what I could do?