I had the same issue with a 2017 iMac 27. At first I was not able to do a clean install of Monterey. Got the same error as above "An error occurred loading the update". Tried everything. Splitting up fusion drive, installing only on blade ssd, only on hd rebuilt Fusion again. Finally I tried taking out one of the ram sticks. No change. Tried replacing the one left in with the first one I took out, and then I finally managed to install Monterey. Installed Office and some other apps. Everything worked fine. Tried installing the possibly faulty RAM, and then the error "Volume Hash Mismatch" appeared. Took out the RAM, and the error never came up again.