You may get this error while you are trying to backup your vCenter as a VM. This happen due to a bug in Veeam Backup and Replication 5.0, specially if you checked the checkbox Application-aware image.
This happen if you are trying to create the Backup job of vCenter VM from from the VC itself.
To overcome this issue, simply;
1. Add the host where the vCenter is running.
2. Delete the previous Backup job that’s pointed to the vCenter
3. Create a new Backup Job and select the vCenter VM from the host where you have added in step 1.
This should solve the VSSControl: Failed to freeze guest