Because this works under a "guest" user account, it implies that MC's configuration or support folders or database, or something is corrupted in some way.
I would suggest the following. Note that what I'm going to suggest is destructive and you might lose your library settings and metadata if something goes wrong. Please understand the procedure and proceed at your own risk. Though it's really not too risky as long as you pay attention.
1. Make a backup of your library: File > Library > backup library Make a note of exactly where you put this backup! You are going to need it.
2. Shut down MC. Make double sure that MC is dead and there are no dock icons of MC that have a dot under them.
3. Remove MC's entire "support folder" which includes the library (database) files that it uses for your main library. To do this, go to Finder and navigate to your Library folder. Press <command><shift>g and in the box that comes up type ~/Library
. Now you will see many folders. You are looking for Application Support. Once you double click on that, find the "JRiver" folder. Remove that folder entirely by dragging it to the Trash, or some menu or right click action that moves it into the Trash.
4. Fire up MC again. It should bounce on the dock for a little bit, as it is recreating this directory structure and some other stuff. It's possible your copy of MC will be unlicensed at this point. You should be prepared to re-license it. This is very easy, I'm just preparing you for this step if necessary.
5. Once MC is up and you can use it, restore your original library: File > Library > Restore library You will need to tell MC which backup file to use, which you noted in step #1 above. Make sure the "Library" and "Settings" check boxes are checked so that it restores everything.
6. You should probably restart MC at this point to make sure everything sticks.
There's some possibility that you will have to redefine your audio devices as well. If so, that shouldn't be very hard. Again, just be prepared for it just in case.
I am hopeful that this "nuke and pave" approach will solve your long standing problem.
Good luck!
Brian.