@Wößner Benjamin - J. Schmalz GmbH, Thanks for posting in Q&A.
To clarify this issue, please check the following.
1.Check whether the user has assigned Intune license.
2.Check the MDM user scope set to All and Windows Information Protection (WIP) user scope set to None.
3.Ensure there not exist device restriction policy or conditional access policy that may block enrolling.
4.Delete the device record both in Microsoft Entra and Intune portal clear the cache and try to re-enroll again.
Please check above information, if there is any update, feel free to contact us.
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