registered parallels 4 now asks me to register

My registered copy of Parallels 4 (which still shows as a registered product in 'my account') is now asking me to register, and, when I attempt to do so, is indicating that there is a problem with an inactive product key (the code is correct). I see that today another user has reported problems with Parallels 5 that are exactly the same:

http://forum.parallels.com/showthread.php?t=103435

Is this a server problem with Parallels? Any help will be very welcome.