Troubleshooting VMware backups
NOTE: The FREE edition of VMware is not supported by NovaBACKUP to perform VMware backups. NovaBACKUP requires a LICENSED edition of VMware vSphere / ESXi to support VM backups, otherwise VMware backup jobs will fail at the start.
First of all it is important to have the latest version of NovaBACKUP installed as the latest version has the most recent fixed in place for VMware VM backups. For instance, v17.2 was the first version of NovaBACKUP to support VMware 6.0 but since then there have been fixed applied for VMware and so it is important to be on the latest version of NovaBACKUP that your license and support package provides. The current version of NovaBACKUP is v18.5.926. You can read about the various VMware related fixes by reading the following Knowledge Base changelog article:
Current NovaBACKUP Version / Changelog
Search for "VMware" to see all of the VMware related fix notes.
Since some of your VM's backup successfully but others don't it seems like the core capabilities of VM backup are in place as far as version requirements go, but it would still be worth upgrading to the Current NovaBACKUP release version since v17.6 is quite out of date at this point and you are eligible to upgrade to it for free with your license and support level currently found.
Since the VM backup utilizes and investigates the VSS - Volume Shadow Copy service inside the VM, you will need to check the VSS status on each VM. You will need to investigate each one of the VM's that are failing to produce a VM backup by investigating the Volume Shadow Copy service on each of those VM's that are failing backup by reading the following Knowledge Base article to investigate VSS on that VM:
Microsoft VSS Troubleshooting:
Please review our KB article Microsoft VSS Troubleshooting
Reboot the PC and start an Administrative Command Prompt and then execute this command:
vssadmin list writers
A list will be produced of all of the Volume Shadow Copy writers and each writer will have a "Status" (current status) line of text. If any of the VSS Writers in the list say "State: Failed" then you will probably need to process a VSS Repair, with this guide:
VSS Repair Strategy #2
You can try to re-register all VSS and COM components by following our FAQ on VSS Command Line Repairs