A Zerto ZVM failing to update on a number of occasions with no real knowledge of why when other Zerto upgrades complete successfully.
The assumption that there is something fundamentally wrong with the installation means it is wise to create a new one and migrate all the setting over.
This is no mean feat, and requires quite a bit of work, but is detailed here: https://www.zerto.com/myzerto/knowledge-base/migrating-settings-and-vpgs-from-one-zvm-to-another/
A change must be raised to perform this installation and migration as it will be service impacting to the DR service.
UPDATE: December 2019
We now have an issue where old history files are not clearing down and we are running out of storage space because of this.
Vmware will not remove the files and it appears Zerto still has hooks in them, despite them not being in use. This along with the impossible upgrades means the only option now is a total rebuild of the Zerto system from a fresh install - probably to new VMs to make absolutely sure that there is no conflict with an old installation. This will mean re-replicating all data - the method above suggests keeping the disks but this would mean a full sync of the disks across sites that would probably take longer and cause more disruption than a straight replication of fresh data, and this has the added advantage of not carrying over any issues that may lie in the original replicated files.