I would guess ( @JorgeB ) could tell for sure that there's problems with the cache drive which may possibly be causing this
g 30 07:54:18 UNBUCKET emhttpd: shcmd (109): mount -t btrfs -o noatime,nodiratime,degraded -U 6bd4f0c7-7a8d-4c23-b9cb-8fbb05f39307 /mnt/cache
Aug 30 07:54:18 UNBUCKET kernel: BTRFS info (device sds1): allowing degraded mounts
Aug 30 07:54:18 UNBUCKET kernel: BTRFS info (device sds1): disk space caching is enabled
Aug 30 07:54:18 UNBUCKET kernel: BTRFS info (device sds1): has skinny extents
Aug 30 07:54:18 UNBUCKET kernel: BTRFS warning (device sds1): devid 2 uuid b450540a-bb2e-4508-96db-e3f32dc9ad66 is missing
Aug 30 07:54:18 UNBUCKET kernel: BTRFS info (device sds1): bdev (null) errs: wr 251066129, rd 74242592, flush 2853347, corrupt 0, gen 0
Aug 30 07:54:19 UNBUCKET kernel: BTRFS info (device sds1): enabling ssd optimizations