This can also happen if your character's profile gets corrupted.
If you keep crashing after a couple of attempts, try navigating down from your documents folder -> EA Games -> Ultima Online Stygian Abyss -> User Data -> (account name) -> (shard name), then renaming the profile for that character and see if you can get in.
( if this then allows you to log in then at least it confirms its something in that file and not on the server; sometimes you can then drill through the XML file you backed up and figure out exactly what's broken, but that depends on your comfort level playing around with these files, otherwise you have to go through and set up your UI for that character over again )