I had this issue.. it turns out old tag sources persist.. the setup doesn't go looking through other user profiles and update their mp3tag application data settings so they do not get overwritten/replaced (not sure if it does normally for the current logged in user).. so if for instance you install mp3tag as "Admin" and then later login as "Geoff" the old tag sources will not be wiped/updated from Geoff's application data folder.
I went to %APPDATA%\Mp3tag\data\sources and saw these files had not been modified in years. As I was not precious about the mp3tag settings for Geoff, I just closed mp3tag, removed the whole AppData's Mp3tag folder and let the application re-create it. It re-created new tag sources and all was well.
Thanks to this page for the heads-up that tag sources are user profile specific : freedb and Web Sources