Turn off
Auto-Import and see if that stops the crashes.
If so, there is almost certainly a corrupt file in your iTunes collection that it is trying to import and getting busted on. If you collect a
Log zip file right after a crash, and post it, I might be able to dig through and find it (though this doesn't always work, depending on when and how the crash happens).
If that doesn't work, you can try importing little bits of the collection at a time (skipping over stuff it already imported, because it almost certainly isn't one of those), to track it down. You'll almost certainly find an obviously broken file that doesn't play in anything (often zero-bytes big, or with other corrupt properties).
If you find one that
does play in other players that causes is, if you can make it available (via a Dropbox public link or something similar) then someone from JRiver will look at it and see if their parser is messed up.