Mp3tag alsways wants to load the files from the last time it was running. I couldn't find an option in the preferences section to deactivate this behaviour. Unfortunately this leads to Mp3tag not starting at all if the files from the last "session" came from an SMB source that's not available any more. In my case this is often caused by a private server which runs only if needed.
and set "Start in this folder" but leave that empty or set it to a folder that will definitly be there.