Hurray! Good to hear that your issue is gone.
Probably some setting from the past, caused by earlier installers or so, caused XnViewMP to not handle an exception in a proper way.
My guess is that the registry delete actions did the trick, but that is really just a guess.
I hope that my testing inspired you and thanks for taking the effort at your end to solve the issue.
I think you can always put back your settings files one by one to see if it causes any problems, if you saved them before and want your settings back.
Having backups of these files is always a good idea, since we all do a lot of tweaking with this highly configurable program.