I have a variation on this problem. Periodically, TMG starts and tells me that I have an expired trial version. I have carefully stored the registration informtion, so I retrieve it and unlock the full version. That's good for a few months, then I get the expired trial version, and I do everything all over again.
Most recently, however, clicking to unlock and then reentering the serial number no longer works. I get an error message. I've double and triple-checked the data that I'm entering and it matches what I have saved and what shows up on "About." What do I do now? Would a reinstall solve my problem?
Here's the weird part. When running the read-only version, if I click "Help" on the menu and select "About" I can see my serial number.
I'm running TMG 9.5 on Windows 10.