Error Code 50 : (client process aborted(50)

Team,

For soem of the client backups are failing with the error code 50.

Please find the detials below :

NB Master server :

OS : Windows 2008 R2

NB Version : 7.5

Client :

OS : Windows XP & Windows 2000.

Backups are going directly to the Physical tape library.

Help in resolving this issue.

let me know if need any more information