OK. So the crashes to mono/sonarr resumed even after the fresh start with 20160918-2. I haven't yet tried 20160920 or 20160921. I was able to isolate the issue to a few video files causing the crashes during series scanning. However, I couldn't determine what was different with them than all my other files that scanned successfully. And oddly, they worked prior to the latest bump in Sonarr version on 20160918. Eventually, I decided to work around the issue by just replacing the offending files. All is now back in working order.
While I was googling and troubleshooting, it did appear that this may be related to an outdated libmediainfo file (v0.7.83) in libreelec master. The current version is v0.7.88. The package in question is located here:
http://LibreELEC.tv/packages/addons/addon-depends/multimedia-tools-depends/mediainfo/package.mk
Not sure if its worth the trouble for you to make the update or not if it was just a few files of mine that were an issue. And it seems no one else seems to be reporting the issue here. Or maybe it should be reported back to the master branch?