Which version of UOFiddler are you trying to run? What's under Details? If it's not latest then try Release UOFiddler version 4.9.12 · polserver/UOFiddler and let me know is the error still there.
Please use latest version first. Then I don't understand what server has to do with fiddler, are you using same files for client+server+fiddler at the same time?I get this same error if I try to use Fiddler while logged into any server, even my personal test server. Makes it a pain having to close out the server, then load Fiddler to find what I'm looking for, then close it out & reload the server. Using Fiddler 4.9.12