Hi ULF Mitch,
Thanks for posting back.
According to the screenshot, the error message may because your organization admin blocks the External sharing settings of Microsoft Forms. Please contact your admin to access Microsoft 365 admin center> Settings> Org settings> under Services, click Microsoft Forms and check the settings. Please turn on these settings. After that, access the form link and check again.
For more details, please view this reference: Administrator settings for Microsoft Forms
If you still can’t access the Edit view of these forms, the issue may be caused by the tenant migration. I would like to suggest you contact your admin to create a support ticket in the Admin Center> Support> New service request. That support team there will have the correct channel to help you investigate more and find what exactly reason has caused this issue.
Appreciate your understanding and patient.
Regards,
George