SinoBreizh

I'm updating this thread just because it can prove useful to someone, even though it has spiralled out of control into troubleshooting hell.
 
I've narrowed the issue down to my RAM. I have two kits of identical RAM, bought at separate times. Both kits have the same model numbers, specs, and timings. Both pass MemTest without issue on their own. Put them together however, and they fail MemTest. After further investigation, I have noticed that, while they have the same model numbers, frequencies, and timings, the older kit of RAM is actually single rank, while the newer kit is dual rank. I never knew the kits were different in this manner (as it is not advertised anywhere on the online shop or the manufacturer's website) and never expected this to be an issue - I had always read that the memory controller would "pick the slower of the two" kits and work from there. I also expected memory incompatibilities to be clear and evident, where the PC wouldn't boot at all; not this unexplainable crash situation. This is the only explanation I can find for otherwise identical kits of RAM passing MemTest individually, but failing when put together, no matter what slots are used.
 
I am currently using only the dual rank dual channel kit of RAM, and I've had consistent uptime with no crashes for around a day now. I have also failed to cause crashes as I did before, by sending a huge 125GB file over SMB. I have even reverted qbitorrent back to LibtorrentV2, with no issues so far. I'll update the post if I spoke too soon, but this seems to be it. For now I'm flairing this as the solution, in case it can help someone.
 
TL;DR: when encountering seemingly unexplainable crashes, running MemTest can be a good first step to narrow the issue down.