Crashed again about thirty minutes ago (after running quietly since, looking back at my last post, October 13, which goes to show what a pain this is to troubleshoot). Guess reseating the memory didn't help after all. I've now updated Unraid to 6.6.4 and tried running that virsh command, and absent other ideas I'll hope that does something and I don't have to look at replacing hardware (probably the memory?).
I was running the Fix Common Problems troubleshooting mode, which writes the diagnostics bundle to the flash drive every half an hour, and also taking a look through the syslog every day or so between my last post and now, and I don't see any errors between then and the crash happening. I'll attach the last diagnostics bundle that was saved before the crash anyway, in case I'm missing something. But as far as I can tell the main thing I have to go on is again some cryptic error screen.
imogen-diagnostics-20181108-0437.zip