Skype for Business unable to set Call Forwarding

Scenario: Call forwarding used to work fine.

Fault:  Attempting to set the call forwarding fails.  The Skype client briefly flashes but does not explain the reason for the failure.

Solution:  User was logged into two workstations with two Skype for Business apps running.  Exiting the second Skype for Business instance allowed the first one to correctly accept a call forwarding state.