FileServerVssAgent idle timeout.
FileServerVssAgent allows only one shadow copy creation session at any time on a specific server. To avoid one unresponsive client holding the file server too long, it maintains a shadow copy creation sequence timer to timeout if the next RPC command is not received within the expected period of time during shadow copy creation sequence. The timer has two intervals: 30 minutes between long client side operations (such as AddToShadowCopySet/PrepareShadowCopySet and GetShareMapping/RecoveryComplete) and 60 seconds between short client side operations.
Check the VSS event log on both file server and application server. Retry and make sure the client side operations are completed with the expected intervals with next RPC command is issued timely.