We are trying to give permission to relate cases to two customers and we can not succeed in doing so.
We know there are pull, transfer, delete, view, and own permissions in the Administration Console. However we can not guess where the Relate Case permissions are. This is the error we are talking about:
The ‘Edit Case’ action permission is required to perform adding ‘Related Cases’ to a case.
The error you are seeing is not because the user cannot perform the action but the user does not have permission to perform action on said case. The user will need to have permission to ‘pull activities’ from the current case owner.