I upgraded my AdvancePro license to a multiple user version however my client machine is not connecting to the server.

When you upgrade your AdvancePro license to include more users please remember to use the latest product key on all client computers also.

Go to Start -->  Run --> Type "REGEDIT" and click OK --> On the left side select HKEY_LOCAL_MACHINE --> SOFTWARE --> ADVANCEPRO --> on the right side delete "Product  Key" record --> then launch AdvancePro and you will be prompted to enter the key again.