After testing 6.9.0 on two other unraid servers for a couple of days, without any problems, I updated the next unraid server, my storage server from 6.9.0-rc2 to 6.9.0.
Now I see drives with a red lock icon, claiming "WRONG ENCRYPTION KEY".
But every time I try to start the array, it is a different set of drives. Even managed to get the array started with all drives unlocked once.
The two other unraid servers also have their drives encrypted, the same way, the same key. No problems there.
Example array start: