VSS backup fails to create snapshot with "VSS_E_SNAPSHOT_SET_IN_PROGRESS" error message when two or more VSS backup jobs run at the same time on the same client.
According to Microsoft, the message means the creation of a shadow copy is in progress, and only one shadow copy creation operation can be in progress at one time. Either wait to try again or return with a failure error code.
There can only be one shadow copy created on a system at a time, independently of whether the jobs attempt to snapshot different drives, this is a Microsoft design/limitation.
Reconfigure backup job schedules so only a single VSS job runs at any given time on the same client.