You could:
Repair the permissions
Reset the computer's SMC and/or PRAM
Or:
Delete the "com.jriver.MediaCenter20.plist" file from your preferences:
Close the Media Center app
Change to your Finder
Chose the "go to" menu and press the "ALT" key
Click "Library" that only will be visible, when you press the "ALT" key
Change to the folder "Preferences"
Look for "com.jriver.MediaCenter20.plist" and instead of deleting it, move it to another location, e.g. the desktop
Start the Media Center app.
Some settings might get lost, but no information (plus you still have the original file).
If this doesn't help, delete the new "com.jriver.MediaCenter20.plist" file in the "Preferences" folder and move back the old file.
Or:
Delete the app and re-install it.
This might help too (maybe together with my first idea).
If not, I don't have any more ideas.
Good luck!
Hans