I too got this start up error after updating to Unraid V7.0.0 from V6.12.14.I followed the instructions steps given in the post above but after removing the pool name "cache" and re-adding it, there was no devices to assign to it. All I got were blanks. unassigned. Trying to restart the shares gave the same error.
So what I did out of pure luck and chance was this:
I removed the pool named "cache" and did not add it back.
Then I just clicked "Start" arrays and it started!
So anyone else running into this issue after updating to V7.0.0 can give that a try.
After it restarted it recognized and reassigned all my drives and shares.
Hope this helps someone!