You shouldn't be using the sa account at all. If Veeam requires login and password then, after checking the authentication modes as Paul suggested, create a new account for Veeam and give it the permissions it requires.
sa should remain disabled.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability