I had this issue on one of my backup jobs recently and I was able to fix the issue with the solution explained below. After a quick google search, I realized that I'm not the only one and neither you are :)
Full Error Description;
Failed to prepare guest for hot backup. Error: VSSControl: Failed to prepare guest for freeze, wait timeout 900 secError: VSSControl: Failed to prepare guest for freeze, wait timeout 900 sec
This can happen with overloaded servers. When the server you're trying to backup is a busy or handles a lot of services at the same time, the Veeam backup server times out to take a snapshot.
The solution involves increasing the timeout value for the VSS on the Veeam server. The default value is 15min and usually you can get around this issue by increasing this value to 30min.
- Logon to your Veeam server and start "Regedit"
- Find the registry key "HKEY_LOCAL_MACHINE\SOFTWARE\Veeam\Veeam Backup and Replication" and add a new DWORD (32-bit) key with the VALUE "180000" (30 minutes) Decimal.
- Go to "Services.msc" and restart the "Veeam Backup Service" service and try again to run your backup job!