Backup For Scaled Out File Server

Hi  ,

I have created a Scaled out File Server on windows server 2012 R2 Failover Cluster . I have added the account used to backup in the backup operators Group and also full permissions have been given for the all the node machines in the failover cluster .

But whenever I add the volume to VSS Snapshot  I get an error VSS_E_SNAPSHOT_SET_IN_PROGRESS .

Please help me out on this .