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.