There haven been similar errors on both cache devices:
Mar 14 19:45:59 Gravity kernel: BTRFS info (device sdd1): bdev /dev/sdd1 errs: wr 428, rd 482, flush 0, corrupt 0, gen 0
Mar 14 19:45:59 Gravity kernel: BTRFS info (device sdd1): bdev /dev/sdc1 errs: wr 295, rd 293, flush 0, corrupt 0, gen 0
Possibly there's a cable/connection problem on both, like for example if they share a SATA splitter, that or some compatibility issue with that model, but that would be strange.
Also see here for better cache pool monitoring.