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For Citrix Xen, this proxy computer is always a virtual machine residing on the XenServer. As needed, you can deploy multiple proxies on a host to provide load distribution for backup operations.
It's similar to Hyper V where the proxy is installed in the existing enviroment.
I don't believe there are any firewall rules for Xen. At least not specifically required ports like VMware uses.
OK, you'll have to look at it from the OS side. The same command is sent everytime from CommVault to have VSS create a snapshot through the writers. The resulting 'VSS_E_PROVIDER_VETO' means the OS had an issue and wasn't able to create the snapshot. The CommVault software does not have a way to workaround the OS failure.
Can you try increasing the timeout in library propertis to 20 minutes and see if there is any change?
Something like this? https://documentation.commvault.com/commvault/v11/article?p=11635_1.htm