Unable to register hosts: Validation failed: Attached to can't be blank

Running into “Validation failed: Attached to can’t be blank” while trying to register hosts.
I already did some research and the internet pointed me towards: Bug #27280: Unable to join a machine with bonded primary interface - Foreman which is closed since 2 years. But in fact the first interface is indeed a bonding one on said maschines.
What can i further provide to help solving this issue?