I've realized that I'm not getting sent the first email of a discussion group thread, so I haven't seen any of the recent group posts. Sorry! I need to investigate why this is happening.
To answer your question, as the error report shows, your files are not UTF-8 encoded, which is the default for AntConc. So, you need to either encode your files correctly, or load your files via the AntConc corpus manager, and set the encoding to whatever you used when you created the files. If you are in Europe, this is probably Latin 1, but it could be anything.
I hope that helps.
Laurence.