I resolved this issue myself, but I'll leave the post as it is just in case someone has a similar problem an finds this vie search engine. It seems like the actual problem was simply that I had a Codemeter of a college plugged in at the same time (Sequoia 12 license). I didn't think this would be a problem, but apparently it was. Apart from that everything works as expected now
I recently tried to start sequoia 15 on my spare laptop but it didn't work. Although my codemeter was plugged in and showed up correctly in the Codemeter control center, Sequoia prompted me for my SN. Then, after entering it, it said I already used all my available activations. Did anyone else have this problem before? Or is this intended behaviour?
To me this seems like it could be an unwanted side effect of the unification of license authorization in sequoia and samplitude. For samplitude licenses that don't use a codemeter dongle, limiting the amount of possible SN-activations by using hardware dependent computer IDs makes a lot of sense. But since the Codemeter already makes sure that I can only use sequoia on one computer at a time anyway, I would expect that I could use any sequoia 15 installation on any computer with my codemeter, even without any internet connection. In fact this does work with all other "dongled" software I use (using iLok, eLicenser or normal thumbdrive) and I know that it did also work just fine with previous Sequoia versions. I always considered this to be a huge advantage of dongles over SN/Internet-based activations. Apart from that, I wonder what I would have to do if this would be a new computer instead of a spare laptop. I didn't find any option to revoke one of the other activations in favor of the new one either.
Thanks in advance for any help!