Replace/swap both cables, or if using some enclosure swap with another disk, then rebuild the disabled disk, not parity.
https://wiki.unraid.net/Troubleshooting#Re-enable_the_drive
Diags are after rebooting so we can't see what happened, but disk looks fine, suggest replacing/swapping cables first just to rule them out and rebuild on top.
I agree, and there's already a request for that, in the meantime I recommend having array auto-start disable when using pools, so you can always check everything is correct before starting the array.
It's balancing the pool to a single device, you can then add the new one, but the best way of doing this would be a direct replacement, i.e., stop array, select new cache device in the place of the old one, start array.
You have two failing disks with single parity, copy everything you can from emulated disk4 and disk8 to other disks in the array, then do a new config with the remaining good disk and re-sync parity, alternatively you can use ddrescue on disk8 and actual disk4 then do a new config with the clones.
That looks like a correctable memory error, check the event log on the board BIOS, it might have more info and identify the slot, e.g., this is from one of my Supermicro boards:
Since they are exactly the same errors on the same blocks it rules out for any transient memory error, are you absolutely sure there wasn't any unclean shutdown? It's what makes more sense, it could be a disk but those cases are extremely rare and the sectors are not all together so even more unlikely, in any case and IMHO you should run a correcting check and as long as there are no more errors in the future you're fine.