Open CiviCase via Webform with manager role assigned
Finally it is working!
Two where important:

Disable &quot;Enforce Permissions&quot; for the contact field in the webform config, filled with a default value.

Look out for the field &quot;Update Existing Case&quot; on the CiviCRM tab. It was set to a case status value, so it would change an existing case.


Many thanks to @AndyClark and @RayWright!