I run UBB 6.5.5 with spiderfrindly URL.
I try to send a PM to a user with a _ (underscore) in his username. This dont work. I get an error that this user doesnt exists in the database. This is maybe a sideffect from the extra security.

/edit: If I add this user to my address book I can send him over the address book a pm. Very strange effect. How can this be fixed?

Problem fixed. Can be closed. It was my language translation. Somewhere in genric.php I use encoded special german chars and this doesnt work. Strange effect.