Insufficient Privileges after Changing Ownership
You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary.
The ownership change is completed successfully.
This error is unrelated to Change Case Ownership Plus and occurs if the user performing the change no longer has Read access to the case after the ownership changes.
To verify this is the root cause, have the user that receives the error try to navigate to the case after they receive the error. The case will not be accessible and the same error is returned when trying to navigate to the record directly by supplying the object ID (starting with 500).
After incurring an Insufficient Privileges error, a user may encounter a "List has no rows for assignment to SObject" error when clicking back on the browser. This error is also related to the user no longer having Read access to the case after the ownership changes.
Contact your Salesforce System Administrator to review your organization's sharing rules. You may also safely ignore this error if the user no longer needs access to the case.
This is by design.