I have Plex v.1.40.0.7775 installed on my TVS-h874 with 64gb Ram and QuTS hero h5.1.4.2596. It happened 2 times in last weeks, memory usage is too high and QuTS shutdown plex server. It never happened with older plex version.
Same issue with v1.40 on QNAP with 32GB of RAM. Never happened with earlier versions, also no other changes were made to the system besides updating Plex Server. Reverting to latest stable v1.32 helped to avoid this problem so far.