Unmount that Unassigned Device.
Stop the array.
Go to Tools - New Config and keep all assignments.
Before starting the array, reassign disk11.
Make sure you do NOT check the box saying parity is valid, because it needs to be rebuilt. It may only be a little out-of-sync but it should be rebuilt anyway.
Then Start the array to begin parity sync.
If there are any problems during parity sync, post your diagnostics but let it continue pending further advice.
What you should see during parity sync is a lot of Reads on all data disks, a lot of Writes to parity, and zeros in the Errors column.
Parity sync errors are expected, be interesting to see how many.