Check if the user 'sa' exists by running from QA:
use yourDB( use Master to find out the opt for SQL Agent permissions)
select master..syslogins.name as login_name,
sysusers.name as user_name
from master..syslogins inner join sysusers
on master..syslogins.sid = sysusers.sid
It probably was deleted, so create that user or a more specific login.
Also, it looks like your 'sa' account does not have a password. Why don't you use some distinctive account and rid of banal 'sa'? Establish an account with permissions and pass, verify everything, syncronize accounts throughout the servers you use, etc.