I am also not familiar with this error, someone pm'd me in game regarding this, so I thought I'd take a peak.
Sounds like a file corruption, missing library, incomplete install, in any of which cases I would recommend Lachian's approach.
Redownload expatch (Ultima Online Excelsiors custom patch).
Redownload UOSteam.
What version of Windows are you on? There have been past Windows updates where UOSteam was temporarily unusable with similar errors, but it seems like its been a few years since that was the case.
Unfortunately, it has been a couple years since I used UOSteam, but I will poke around the internet to see if I can find anything :/. Sorry to hear you're having trouble. If none of that works, you could try looking into one of the other "scripting" clients like Razor or (if you like a coding language) Orion.
[edit]: There seems to be a few links when I google the error.
The best potential explanation I found was in the following thread.
https://uoevo.com/forum/community/help/uosteam-issues/
"Steam has a built in mapping app which is probably why its looking for the*.mul files. If you open the directory your UO client is installed to, you need to make sure you have the 6 map files map?.mul (where the ? is sequentialy the integers 0 through 5). If I recall correctly the UO client itself no longer needs these files to run, so some installations do not provide these files."
They seem to be suggesting that UOSteam is complaining that your client doesn't seem to have all the files that it should. My UOSteam auto detects my client exe, but maybe you'll need to select it manually. Either way, make sure that you have the map mul files after you reinstall the client, uninstall UOSteam, reinstall UOSteam, reboot your computer, and are still experiencing the error.
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