A 406 error is generated by the Apache Module Mod_Security; it has nothing to do with IE7 or the UBB; it is in fact your host using a highly anal retentive security module that works for crap... Only they can fix any mod_security issues.
Feel free to search here for many discussions on mod_security (as an ex TS of a webhost, I tend to know about these things, the mudayne information you keep in your head..)
