JorgeB

There's nothing relevant logged, and the server still crashing in maintenance mode points more to a hardware issue, could just be overheating, or power related, since a rebuild will use more power/cause more heat. Reformatting the emulated drives will delete all data there, and IMHO nlikely that the disks are the problem, I would check CPU temps and/or try again with a different PSU, if available.

Check filesystem on disk4, run it without -n, and if it asks for it use -L

Yes, and my suggestion was to try the above to see if the issue returns or not, or wss this a one time thing?

I would recommend resetting the current stats and monitoring the pool for future issues, since it can be much harder to resolve a problem if a bad or dropped device goes undetected for some time.

Please post the diagnostics.

More zfs related crashes, I would recommend backing up and re-formatting the zfs pool.

If you really need Wi-Fi why not use an AP with client mode?

I would recommend updating, it's not complicated: https://forums.unraid.net/topic/97870-how-to-upgrade-an-lsi-hba-firmware-using-unraid/

You should delete those files if possible, though don't know if it will affect Plex.

Look in syslog for a list of corrupt file(s), those should be deleted/restored from a backup, then re-run a scrub to confirm there aren't any more errors.

That won't work, abort and run it again using the GUI, but without -n.

Try a different flash drive, just to test, no key needed, also look for a BIOS update.

Possibly, could still be a RAM issue since memtest is only definitive if it finds errors, since you have multiple sticks try with just one, if the same try with a different one, that will basically rule out bad RAM.

Yes.

Try running another filesystem check, that still looks like a filesystem problem to me, if it doesn't help, I would backup and re-format that disk, to make sure the issue is resolved.