Found the problem....
the query parameters of the rss url presented by the forum are separated by a ; however anubis doesn't understand that. It is also not seen as one name=value, since matching rss on value doesn't match eiter. I've tried with some server rewrites to change the first separator into a & but failed. So for me the only solution it to replace all 10+ locations where this url is presented.
So the get an rss feed, replace the ; with an &
Sigh....