OK, I waited and waited for an expert to reply, but since no one has answered, I guess the knowledge I have might help.
PLEASE KNOW.. I'm no expert...
That said, I'll tell you how I dealt with the error.
First off, I think the error occurs when UO Fiddler is unable to either locate items it expects to find, or finds item it has no idea how to deal with.
I received the error every time I searched, modified, and opened an item. I got the same error, but I selected CONTINUE and not quit. This allowed me to keep the program running at least. Secondly, you'll need a way to open the UOP files since the can't be read by the current UO Fiddler. I used
LegacyMULConverter by Eos, but you can choose the program that will extract the files in a format that Fiddler can read. LegacyMULConverter will only handle artLegacyMUL.uop, gumpartLegacyMUL.uop, soundLegacyMUL.uop or mapXLegacyMUL.uop (where X is /[0-5]x?/)... Please refer to the disclaimer I wrote saying I'm no expert... I found no issue having the UOP files and the MUL files both in the UO directory, but that was me. Make sure you read the doc's on this. I was able to modify my client, 188.8.131.52 without issues. I was able to modify my client with the stalagmite, the colored ground twigs, and trap patches, the spider web removal patch, trees to stumps, as well as various animation patches. I also changed the radar, and the journal patches work with out issue. Needless to say, I had TONS of reading before I was able to accomplish this, and backed up EVERYTHING Numerous times. But in the end, I have a modified client, and everything works fine. Believe me though, it was well worth it... Hope this helps, and I hope the EXPERTS comew out of the woodwork for you ... GOOD LUCK