Hi,
At my previous customer we were facing the same issue. All earlier upgrades went smoothly but the upgrade to 2.18 gave us the same issue! Another schema was used (not dbo), we opened a SR but after 2 weeks there was no progress in this case. So we built 4 new appvolumes managers and imported the appstacks and added these to our loadbalancer and we removed the old managers. This was for us the fastest path to 2.18.