I think one of the reasons for this error could be that someone has either deleted a snapshot from the SnapInfo volume or trying to re-connect to the SnapInfo LUN. Under any circumstance, snapshots are missing in the usual sequence of snapshots that SMSQL expects. Please ensure that the SnapInfo volume is not full.
To recover from this error, delete all previous backups taken by SMSQL from the GUI itself and then try to take a fresh backup. Let me know if that too fails.
Do your databases volumes reside on a storevault box?