Could you check whether MP3tag has internet access at all?
Try Help>Check for new version.
You should get a message box.
If that does not appear, then MP3tag has no internet access and you have to find out what is blocking MP3tag in your setup.
No, it was working at the time of you posting (and there would have been far more reports). It's possible that your IP address, IP range, or VPN was temporarily blocked by MusicBrainz.