When using Veeam Backup & Replication for an application aware image processing backup of the virtual machine which hosts the SQL database used by VMware vCenter Server, the backup will fail with a VSSControl: Failed to freeze guest, wait timeout error. This happens when Veeam communicates with vCenter Server for creation of the VSS snapshot. It is a kind of deadlock where Veeam B&R wants to communicate with vCenter while at the same time the database is frozen because of the VSS-snapshot.
To solve this problem, add the IP-adress of the ESX host which runs the vCenter Server SQL database to the Veeam console (the servers list in the left panel). Then adjust the job and select the virtual machine with the SQL database from the ESX host instead of via the vCenter Server. Veeam B&R will then use the host for communication with the VM to do the VSS snapshot.
You have to make sure the SQL VM does not move to another ESX host. Use a DRS Host Affinity rule to tie the SQL VM to the particular host which was added to the Veeam console. As a best practise the VM running vCenter Server should as well be pinned to a certain server so you always know where it is running in case of problems.