Ok, now its playing long .mp4
Short .Mp4 10--40 seconds long, crash immediately
Suspect that was the problem all along
Cannot load playlist with short Mp4:s, it crashes
There might be an issue with your short mp4's.
At any rate, you remove the MC App by dragging it from the Applications folder into the trash.
You remove the user entries to remove the rest by deleting this folder:
~/Library/Application Support/J River/Media Center 31
Note that the ~ is shorthand for starting with your user directory so if your user is Korppi it would be
/Users/Korppi/Library/Application Support/J River/Media Center 31
When you install again then you'll start fresh and need to either restore the library from a backup or reimport all of your media.
You will have to license again and there is no trial period for reinstalls. The easiest way to reinstall the license is to copy and paste it from your registration email into the dialog that will come up the first time you run after the reinstall.