Checking the original diags to refresh my memory on what happened here I just noticed that disk8 failed to mount even before there were read errors during the rebuild:
Feb 5 22:47:43 TC-NAS-01 emhttpd: shcmd (184): mount -t xfs -o noatime,nodiratime /dev/md8 /mnt/disk8
Feb 5 22:47:43 TC-NAS-01 kernel: XFS (md8): Mounting V5 Filesystem
Feb 5 22:47:43 TC-NAS-01 kernel: XFS (md8): Log inconsistent (didn't find previous header)
Feb 5 22:47:43 TC-NAS-01 kernel: XFS (md8): failed to find log head
Feb 5 22:47:43 TC-NAS-01 kernel: XFS (md8): log mount/recovery failed: error -5
Feb 5 22:47:43 TC-NAS-01 kernel: XFS (md8): log mount failed
Feb 5 22:47:43 TC-NAS-01 root: mount: /mnt/disk8: can't read superblock on /dev/md8.
Feb 5 22:47:43 TC-NAS-01 emhttpd: shcmd (184): exit status: 32
Feb 5 22:47:43 TC-NAS-01 emhttpd: /mnt/disk8 mount error: No file system
Feb 5 22:47:43 TC-NAS-01 emhttpd: shcmd (185): umount /mnt/disk8
Feb 5 22:47:43 TC-NAS-01 root: umount: /mnt/disk8: not mounted.
This suggests there were already filesystem issues, so you can still continue but success depends on how bad that corruption was, but it might be easily fixable by xfs_repair, everything else looks fine for now, the procedure is:
-Tools -> New Config -> Retain current configuration: All -> Apply
-Check all assignments and assign any missing disk(s) if needed.
-Important - After checking the assignments leave the browser on that page, the "Main" page.
-Open an SSH session/use the console and type (don't copy/paste directly from the forum, as sometimes it can insert extra characters):
mdcmd set invalidslot 8 29
-Back on the GUI and without refreshing the page, just start the array, do not check the "parity is already valid" box (GUI will still show that data on parity disk(s) will be overwritten, this is normal as it doesn't account for the invalid slot command, but they won't be as long as the procedure was correctly done), disk8 will start rebuilding, disk should mount immediately (it likely won't mount in this case) but if it's unmountable don't format, wait for the rebuild to finish and then run a filesystem check