BackupExec requires deploying an agent on each guest. This agent also brings custom VSS provider that "brakes" normal VSS operations unless I assume you do some special custom calls that their agent does. Veeam leverages standard VSS API calls, and those are affected. This is known issue, there were a few other topics about this over the course of last couple of years. Uninstall + reboot always helps.
I figured this is Symantec's last revenge to us before they get thrown away