Hello @stefan ,
Please check couple of settings in cluster properties: Are you seeing this value as Unchecked? Use network name for computer name
If so, verify the following registry entries in both nodes
If this is presented, you need to manually delete the entry.
Actually, what happens is when the settings is unchecked, the value is not deleting from Registry.
Please mark as "Accept the answer" if the above steps helps you. Others with similar issues can also follow the solution as per your suggestion