limetech's post in Cache Pool - Prefer - Greyed out / Unknown Encryption State was marked as the answer
We can add a fix for this in 6.10, however a workaround for you is to Stop the array and then edit the file:
find this line:
and change to:
You can edit the file by navigating to it via 'flash' share on your network, or open Terminal window and directly edit on the path:
Set the value exactly to all lower-case "no".