Hi, I'm currently struggling a bit... First I can't find the serial key number again. I'v bought the App 26.08.2024. I just have an email from Apple.
Second, since I'v worked on the metadatas of my music librairy in Rekordbox I can't see all those informations again in Mp3tag 
Actually for one half of ALL my tracks: informations like Artist, Title, Year, Comment, completely disappeared...
I can see everything in Tag Editor Lite but not with MP3Tag anymore 
Thank you for your help!
If you've bought the app via Apple and the Mac App Store, the licensing works without a registration code. To check make sure you're logged in at the Mac App Store app with your Apple account. You can check if the icon on the lower left shows you name.
If it doesn't show your name, you'd need to login via ⌘0 where you can also see all of your purchased apps and can download Mp3tag via the respective icon.
I've sent you a PM with information how you can share one of the tracks with me.
Just wanted to give a quick update here: the example files had no problems and I could read all metadata on my test system.
This pointed in a different direction, a limit of ~3000 files opened individually (a bug in macOS's sandboxing). A quick back an forth showed, that even loading a folder (not the individual files) wasn't possible anymore.
Once the Mac was rebooted and all resources were freed, opening the parent directory of all the files made all metadata reappear.