I'm assuming we're talking about the classic (2d) client here.
Code:
[drive letter]:\Users[depending on how you have your documents folders configured]\[Windows user account name]\My Documents\EA Games\Ultima Online Legacy\User Data\[UO account name]\[shard name]\[character name]
That folder contains the files for your character. You can safely delete any of them when you are logged off and they will simply be recreated. However, you might want to back up that folder if you have a lot of macros etc. I'd try deleting the "Multicache.dat" file first, then the "uo.cfg" and see if one of those two fixes your problem, before deleting the other files which might mean a lot of work setting up your desktop, macros, etc. again.