So if you have problem that goes away with another user, the problem commonly lies in a corrupted preference related to the app, so you start hunting out the .plists for the application and moving them to the desktop before relaunching and seeing if the problem is fixed.
If not, then you start poking about in Application Support for the app.
After 9 years of Apple use, I have successfully made this method work one time (iMessages), and even then, it took a full system reboot after each file move to validate whether or not that had made a difference.
The whole process took the better part of a day. It was long, it was tedious, but at least I got there in the end. There was no paid-support story that would have done this for me, but hey ho, it all worked out. I can't imagine I'll ever have the time to go through that much effort again.
The same cant be said for my iMessage history on my iPhone, which has been lost repeatedly as the only solution was "don't restore from your backup".
If not, then you start poking about in Application Support for the app.