I've had XP11 since the early beta days and recently updated to the newest version with no issues.   The other day, I was asked to unlock X-Plane which I tried to do with my product key.  When I tried to do so, I was told that the "Peer certificate cannot be authenticated with given CA certificates."  I contacted X-Plane support and they promptly responded that it was a firewall issue.  I'm using the latest version of Windows 10 (Creators Update) and disabled the Windows Firewall and my anti-virus with the same result.  I tried booting into safe mode but XP11 won't start in safe mode.  

Has anyone else run into this issue?

That is very odd, I have the same version of windows with active firewall and windows stock anti-virus and no issues. There have been a couple of times in the past where the updater does ask to re-verify key, but had no problems.

I can't think of a suggestion right now, sorry Rick.

Thanks - I'll keep checking around and will come up with the answer.

I performed a clean boot of Windows 10, opened X Plane 11, and the verification was successful.