All music disappears when opening iTunes Library.itl backup file

I've been using Apple Music for years, but have recently tried out Spotify. I didn't like it and re-subscribed to Apple Music to find my Library of 10 Years GONE.

I have backups of my Windows PC, from which I got a recent iTunes folder with the iTunes Library.itl (and additional files). From everything I read, I should be easily able to recovery my library by Shift-Opening (Win) or Option-Opening (Mac) my iTunes/Music and selecting my iTunes Library.itl.

And this works, when I open it I get my whole library which consists 95% of Apple Music Tracks and one local Album (which I also still have) and I can listen to all of them too.

Now here comes the Problem:

When I try to sync this library to my other devices by clicking "Update iCloud Music Library", all tracks instantly disappear expect my one local album and I get the following message:

When I first try to disable genius, by deactivating iCloud sync in the settings (which is always automatically activated when I open the .itl), the same things happens and this message appears and all my music is gone. Same goes when trying to "Update Genius".

Other things I've tried:

1. Option-Opening the .itl on my Mac -> Library Opens for a second, and then everything disappears except the one local album, very similar like on Windows, but instantly and without error message.
2. (Win and Mac) Try it without internet connection -> Same behavior.
3. (Win and Mac) Try to disable "iCould Music Sync" before opening the .itl -> When opening the .itl, the iCloud Music Sync always comes on.
4. (Win and Mac) Closing and opening iTunes/Music after opening the .itl, but before trying to sync -> Same behavior.
5. Trying "Organize Library" -> Doesn't do anything.
6. Clearing my library and disabling iCloud Music Sync on all other devices and deleting the Apps, as to not interfere with the Sync. -> Doesn't help.
7. I don't have any other backups like Timemachine or iCloud Backups for iPhone.
8. (Win and Mac) Try to "Export Library" before its gone, and Import the xml export in a freshly created library -> This kind of works, I get my whole library but all my tracks are greyed out and have the status "Waiting". Clicking "Update iCloud Music Library" or enabling/disabling iCloud sync doesn't help. The tracks do indeed sync to all other devices, but are greyed out everywhere.

I'm at a loss with this, I've curated this library for over 10 years. First my music is just GONE and then my backups don't even work...

Does anyone have any suggestion what else I can do?