Hi everyone,
I had a call with Microsoft. It seemed like Azure had a problem with known users (who where once synced to Azure) and syncing them again in a new OU.
So when I first placed them in the custom users OU and synced them, it began to work.
Placing them back and syncing them, worked too.
And I've tested a new user in the new OU and that object got synced too. So it works!
Kind regards,