I decided to buy the program, restarted it and of course, now I don't see the problem!
I suspect this may happen when the prgram starts at boot time. For the moment my PC is busy with a backup, but once it's done, I'll reboot it and see if I can make it happen again.
FYI, I use custom scaling of 135% with Windows 7 x64. It's likely the problem is due to the custom scaling.