@Neubauer, Lukas Thanks for reaching out and Apologizes for delay on this.
If you are facing this while doing a hybrid Azure AD join on a machine, this is part of the process where the device join process waits up for the onprem computer object to be synced to Azure AD using AAD connect and then the Device registration service should be able to add it.
The whole process should be done within 30 mins or so, are you still having issues with this ?
If yes, you can start the troubleshooting from this event viewer : Open the User Device Registration event logs in event viewer. Located under Applications and Services Log > Microsoft > Windows > User Device Registration
Let me know if you need any help, we can take this offline to have a look as well.
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