@Stephan Avanozian Thanks for reaching out.
IF the device is on prem domain joined and you are trying to remove it from domain, then you would need to know the local admin password which you can provide while disconnecting the machine from domain.
At that point it would not take any email address. So try with the local admin password once.
If you still face the issue, let me know we can sync up offline to do a cleanup and start fresh.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.