I checked with my colleague and it appears to be an issue with VSS, the Windows system files, or general system configuration. He mentioned the server was very slow and VSS reported flush timeout errors. In case other readers experience similar issues, we have a VSS troubleshooting guide and other helpful articles in our support section
Unfortunately, to our knowledge, the VSS timeout cannot be changed; hence, if after careful investigation this is found to be indeed the issue you may want to find ways to speed up your PC and reduce the load during backups. Perhaps shutting down other apps may work
Hope this helps!