"wtf, why?!"
Thanks to the autor of this reddit-post:
https://tinyurl.com/y9yaqwqp
This error seems to appear only on Mac. It means, that some DRM-stuff is broken and needs to be recreated.
Fix:
- Boot into recovery (CMD+R)
- Open the built-in terminal and type in the command EXACTLY as written:
nvram 8be4df61-93ca-11d2-aa0d-00e098032b8c:epid_provisioned=%01%00%00%00
- press Return
- type in reboot and press Return again
Now everything should work again.