Message logs in the Management Console of the cloud failover appliance (CFA) show the following error:
VSS Snapshot Creation failed. Check Log file for more details.
The VSS failure you see is due to insufficient storage space available to create the VSS snapshot.
The actual error message in the trace file is the following:
Steps to resolve
systeminfo from the Support tab, 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 knowledge base article for details.