I had a a similar problem, so I'll share what worked in case anyone else is searching. Apparently, the license was already activated (I had clicked the link before and the license was already tied to my account via the web browser management interface). When things weren't working in the TeamViewer PC app ("trial period ended"), I tried clicking the link in email and got the "invalid license number" error. The problem was that my PC application wasn't linked correctly to my account, even though it was in the list (the only thing in the list) in the management web interface. I removed my PC from the device list there, then closed the PC app (fully - so it's not even running in the background), then opened it and logged back in. This connected the PC app, and it again shows up in my device list with the problem solved. Hope this saves someone else some headaches!
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