iTunes does not allow me to add anything to the library

Since some time I have my iTunes library due to the size on a NAS storage - as the storage had a failure, it was exchanged and all data were migrated to the new NAS. Now I suddenly got the following problem with mit iTunes Library:

I have NO access problems to the migrated library (no name change, just different HW), everything is there and I can open iTunes and see my complete library and play all of its contents without problems. But:

I can not download anything from the itunes store, my iTunes freezes immediately and does not even start the download (no error code - it dies before)

I can not change any properties of the files (such as the video information) - if I do that, iTunes freezes immediately.

I can not add any covers to videos, iTunes freezes immediately

My Conclusion: I can only read my library, but not write on it, although I do have reading and writing rights on the NAS, and it even is the same, if I use admin access for the storage. My Mac is authorized for my iTunes library.

iTunes can only be closed by force in all cases.

My iTunes version is: 12.9.2.5 and my Mac is running on macOS Mojave, version 10.14.3

I have already tried to exchange the itunes library.itl - but no change to the situation.

Do I have to entirely re-install iTunes and re-add all my videos and music into a entirely new library resp. download it again from the iTunes store, or is there any other solution possible. HELP would be highly appreciated, as the first option would take me weeks.