It looks like your disk(s) have a lot of btrfs errors:
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): forced readonly
Dec 9 01:50:08 Highlander kernel: BTRFS: error (device sda1) in btrfs_finish_ordered_io:3074: errno=-5 IO failure
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14581760
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14585856
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14577664
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14594048
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14577664
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14598144
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14581760
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14602240
Dec 9 01:50:08 Highlander kernel: BTRFS info (device sda1): no csum found for inode 4647604 start 14581760
Dec 9 01:50:08 Highlander kernel: BTRFS: error (device sda1) in btrfs_finish_ordered_io:3074: errno=-5 IO failure
Dec 9 01:50:08 Highlander unassigned.devices: benchmark: shell_exec(/usr/sbin/hdparm -C /dev/sda 2>/dev/null | /bin/grep -c standby) took 106.512354s.
Dec 9 01:50:08 Highlander kernel: sd 0:0:0:0: [sda] Synchronizing SCSI cache
Dec 9 01:50:08 Highlander kernel: sd 0:0:0:0: [sda] Synchronize Cache(10) failed: Result: hostbyte=0x01 driverbyte=0x00
Dec 9 01:50:08 Highlander kernel: BTRFS: error (device sda1) in btrfs_run_delayed_refs:3058: errno=-5 IO failure
Dec 9 01:50:08 Highlander kernel: BTRFS error (device sda1): pending csums is 21020672
Dec 9 01:50:08 Highlander kernel: usb 3-1: new high-speed USB device number 3 using xhci_hcd
Dec 9 01:50:13 Highlander kernel: btrfs_dev_stat_print_on_error: 12786 callbacks suppressed
Dec 9 01:50:13 Highlander kernel: BTRFS error (device sda1): bdev /dev/sdb1 errs: wr 10555024, rd 11332069, flush 304, corrupt 81, gen 1
Dec 9 01:50:13 Highlander kernel: BTRFS error (device sda1): bdev /dev/sda1 errs: wr 1, rd 7704, flush 0, corrupt 0, gen 0
Dec 9 01:50:13 Highlander kernel: BTRFS error (device sda1): bdev /dev/sdb1 errs: wr 10555025, rd 11332069, flush 304, corrupt 81, gen 1
Dec 9 01:50:13 Highlander kernel: BTRFS error (device sda1): bdev /dev/sda1 errs: wr 2, rd 7704, flush 0, corrupt 0, gen 0
Dec 9 01:50:13 Highlander kernel: BTRFS error (device sda1): bdev /dev/sdb1 errs: wr 10555025, rd 11332070, flush 304, corrupt 81, gen 1
Dec 9 01:50:13 Highlander kernel: BTRFS error (device sda1): bdev /dev/sda1 errs: wr 2, rd 7705, flush 0, corrupt 0, gen 0
Dec 9 01:50:13 Highlander kernel: BTRFS error (device sda1): bdev /dev/sdb1 errs: wr 10555025, rd 11332071, flush 304, corrupt 81, gen 1
Dec 9 01:50:13 Highlander kernel: BTRFS error (device sda1): bdev /dev/sda1 errs: wr 2, rd 7706, flush 0, corrupt 0, gen 0
I can't make any sense of your log with regards to UD, but it appears you have disks with a lot of errors. If the SSD (sda) is on a USB port, you may have port or cable problem.