OK, non-intuitive solution (at least for me). I opened a non x-aviation aircraft and still got the popup box. But this time, it accepted my authentication and, apparently, everything is back to normal. So I'm back in the Saab!
I am now experiencing the same difficulty opening up x-aviation products in xp10. I have tried cut and paste, manual entry, and changing my password at the x-aviation site but no luck. I have a bunch of plugins running but haven't tried shutting them down yet. Is that next step?