Hey! I also have had two odd crashes in the last week with my previously-rock-solid-for-6+-months server.
Only things that have changed:
1) Updated the firmware on my cache pool 2x980 PRO SSD's (not using this Cache Pool for anything Plex, but it DOES have the appdata on it for Docker...)
2) Updated Plex to the newest version (as of like a week ago, I noticed there is an "update available" but I need to get numbers - currently at work so I'll get those as soon as I can.
Both crashes were very strange. Like, the server didn't fully reboot, but the array was stopped and it noted UNCLEAN SHUTDOWN. After the first one, I started everything back up, and did a parity check. It ran for about 20hrs then randomly crashed again...but weirdly, it sent a notice that "parity check finished" despite still having about 8hrs left on it. So, something is doing a weird..."soft" reset of some kind?
I did see a MCE in the SYSTEM LOG but need to look at it more and probably make my own post with my DIAGNOSTICS.
For anyone to help on here, they'll ask for your DIAGNOSTICS so you should add that to your OP when you have time.