Nov 28 03:50:23 valhalla kernel: BTRFS error (device sdb1): bdev /dev/sdb1 errs: wr 248, rd 778, flush 0, corrupt 0, gen 0
This is being caused by
Nov 25 03:46:18 valhalla kernel: ata1.00: failed command: WRITE FPDMA QUEUED
Nov 25 03:46:18 valhalla kernel: ata1.00: cmd 61/00:50:a0:61:bc/04:00:00:00:00/40 tag 10 ncq dma 524288 out
Nov 25 03:46:18 valhalla kernel: res 41/40:48:08:4a:af/00:00:19:00:00/40 Emask 0x9 (media error)
Which is in turn related to either cabling to the cache drive (loose?) or alternatively
197 Current_Pending_Sector -O--CK 100 100 000 - 64
You can try running the extended SMART tests against the cache drive to see if it clears this stuff up.
On the other hand, since you only have a single cache drive in the pool you're going to have best results by reformatting it as XFS instead of BTRFS. BTRFS has tendencies to not be very forgiving in certain situations (or it is buggy) whereas XFS is rock solid and can handle any weirdness
Converting will require you to stop all services (docker and virtual machines) from the settings tab and then moving everything off of the cache drive onto the array temporarily (set all the shares to be use cache:YES) and then running mover.
After everything is finished, change the format of the drive to XFS start the array and set all the applicable shares to be use cache: PREFER and then run mover. Afterwards, you should be able to (hopefully) re-enable the services and you're back in business.