Greetings, thank you for the quick reply!
I discovered, that increasing the bcrypt_rounds was slowing down the server so much, that it could not save the Admin password properly on first run. So it never had an Admin to refer to. Once I removed the env var for increasing the bcrypt_rounds (defaults to 12), everything went smoothly, as it should.
Not sure if this is a bug with NodeBB not handling increased bcrypt_rounds, properly.