I have had MC9 unload itself in two distinct situations:
(1) I upgraded one of my PC's from MJ8 to MC9 with the uninstall of MJ8 and removal of '\program files\JRiver\etc.' When MC9 loaded and when I used Tools\Options\Encoding and selected MP3, the program required a net install of the encoder. Upon completion, MJ9 exited abruptly (though MP3 encoder seemed to have been installed).
A complete scrub of the registry of all 'JRiver', 'MediaJukebox', and 'JRiver' entries seemed to make the program happy the next time it was installed. I tested this by uninstalling MC9 and scrubbing the registry then reinstalling the program, this time, it behaved well.
It seems a more complete uninstall may be called for (all but the registration registry entry)?
(2) the second instance of unexplained program termination was more subtle: I was experimenting with adding fields to the overall database for extended genre definitions and other sorting functions. When I cleared the library and then did an import, the program would get about half way through the process when suddenly an hourglass icon would appear on the cursor pointer and a minute later the program would exit. My library is only 16,000 tracks and I now know that is nothing compared to KingSparta and others. As my "mediafiles.jmd" file is only 12,577 mb, DB file size was not the issue.
I traced the problem using successive to a folder of Schubert lieder which had many tracks named with foriegn characters (since it was German, mostly umlots). There were 12 folders with about 20 tracks each. When I substituded a plain vowel for all with accents, the problem went away and now the entire library re-imports with no further problems.