As the time Symantec Support solved two other issues with permissions, asset merging is now magically working.
We found the following:
If you want to be able to put assets in organizational views, one must have all system permissions, especially "change permissions", which I never thought of and all ressource permissions for the source and destination organizational group.
The second: If you want to see CMDB Functions->Set Asset Status for an asset, one must have the "delete" permission on that object.
Maybe by accident, we also repaired the asset merging with that, too. The other possible idea is, that we currently don't have an asset with dataclasses that throw the error and we will run in the problems again with other assets.
To prevent a user actually to use the "delete" permission, I removed all permissions for the "Delete" from right click menu.
Best regards,
Andreas