If a Client device has checked a license out of an ExtendSim 9 Network license and cannot check it back in, they should be presented with an error message. If that error message states it is an undefined license, the registry keys ExtendSim uses to identify the license files has probably been trashed.
To resolve this issue, on the Client device, remove the lic.dat file from the Documents > ExtendSim9 folder. This will reset the Client device so it will no longer be checked out allowing ExtendSim to launch on that device and connect to the Server as it should.
This procedure will enable the Client to use ExtendSim, but it does not repair the missing license instance for the License Manager. It will still show the license count is down by 1. See the FAQ ExtendSim 9 Network Issue - Resetting the license count for the License Manager.