Unable to create snapshot The error message was: 'VssSyncStart' operation failed

Dear All,

               we are try take VMDK backup, after firing the backup we got 'VssSyncStart' error in vcenter...

           we try to upgrade vmware tools and uninstall reinstall vmware tools still we are facing same issue.

          Esxi ver:-5.1 u1

          vCenetr :-5.1 u1

               Please provide solution..

Thanks &regards

Srikanth Raavi