I ran into the same problem some time ago (half a year?) because the beta period of the free trial essentially ran out.

I then contacted Magnus, and much to my surprise he responded quite quickly. He told me he had no plans of further supporting MP3TagStudio, because ever since the rise of streaming services like Spotify, a lot of people didn't bother downloading and maintaining an MP3 collection anymore. That simple fact showed bigtime in the registration numbers of MP3TagStudio, which apparently had plummeted. As a means of "thank you" to Marcus, I registered the program right there and then (something I should have done years ago). I received a registration and nowadays I'm still using MP3TagStudio on a very frequent basis, on Windows 8.1.

I guess I wrote up all this just to say: contact Marcus and ask him if he can help. It seems he still gets and reads the emails and he seemed like a nice enough guy. I guess it can't hurt to try.