I noticed yesterday that I was getting a patching error of some sort when I tried (and failed) to start a second instance of UO running while I already had it running. The first copy of the client that I had open had received yesterday's patch and everything was running fine in it. I just couldn't seem to start up another copy of the client to hand some stuff from one character to another. I even tried starting the second copy using just the client.exe file so it wouldn't attempt to patch, but that didn't work either.
I ended up closing the first copy of the UO client and starting all over again. Started up one copy of the client (using the uo.exe file, since I don't use UO Assist) and after logging in on that started up a second copy of the client, also using the uo.exe file. Both opened just fine. So if you're trying to open a second copy of the client while still running a just-patched copy of the client, you may have problems and need to close that first copy. Maybe that first run of the newly patched client has to be shut down properly and save some changes before you can try running a second instance of it. Not sure this is what you're running into, flappy, but thought I'd throw it out there in case that's what's happening to you.