WoW has file named scan.dll.new that contains a reference list of common malware programs that might be used to attach to the game files. However, it was flagged at one point by Norton 360 as a keylogger, which was quickly reported by users. It is in fact not malware itself nor is it part of WoW's Warden anti-cheat software.
UO might have a similar type of file, but I'd suspect that you in fact have malware that has infiltrated the game files, if no other UO players who are also users of your AV product report a similar occurrence of a verifiable game file being flagged as malware.
Did your AV reference the name of the file? That would be the place to start.
-Skylark