Attempting to return borrowed licenses from ArcGIS Administrator returns the following error message:
Error: Unable to return licenses
The error occurs when untrusted licenses are returned to the license server. A borrowed license becomes untrusted when licenses from ArcGIS License Server Administrator are transferred from one server machine to another before the borrowed license is returned.
Clear the trusted storage of all ArcGIS Desktop licenses. Refer to How To: Clear or delete trusted storage for concurrent-use and single-use licenses for steps to do this. The borrowed licenses are removed from the client machine.