You have not given us anything to work with.
Tell us more about the type of backup - Filesystem? Database?
Is error seen with one client? or more than one client?
Has this backup worked before and is now failing?
Or never worked?
Is error seen right at the beginning during connection to client?
Have you confirm port connectivity between client and server?
Have you confirmed forward and reverse name lookup between server and client? (DNS or hosts files)
Show us all text in Details tab of the failed job.
As per above post:
ALL support for NBU 6.x ended in Oct 2012. You really need to upgrade!