I just remembered a similar post, which I think is the only other time someone's seen this. (Not certain it's the same issue but it sounds similar.)
resource.dopus.com/viewtopic.php ... r+checkbox
I think a settings backup (use the Settings > Import & Export... wizard so you get everything), then a complete uninstall/clean (e.g. delete the GPSoftware registry keys under HKLM and HKCU if you're familiary with registry editing), then a reinstall is the best thing to try.
Check that the problem is gone before importing the saved settings, in case something within them is the trigger.
Let us know how you get on, and if the settings file triggers the problem then it's something GPSoft would probably like to take a look at.