VSS Snapshot Creation failed. Check Log file for more details. # or Warning: Plugin VssPlugin do_VSS failed. # or Warning: Plugin VssPlugin Post VSS failed. # or Running as 'SYSTEM'. Privmask=00000206 # or VSS PLUGIN start backup job # or Warning: Plugin VssPlugin finish_VSS failed.
The VSS failure you see is due to insufficient storage space available to create the VSS snapshot.
The actual error text in the trace files is
Error text: VSS_E_INSUFFICIENT_STORAGE.
Steps to resolve
systeminfo from Diagnostics and you can see the available resources for the server.
If this is a VM, you may be able to devote more system resources to that server and get away from the error.
See Microsoft Docs for further details.