SQL Server backup failing. Error: 3041, Severity: 16, State: 1
As Max pointed out, the sa login error doesn't corrolate to the backup time as it happens several hours before, however the 2nd one does.  Have you looked into:
http://blogs.msdn.com/b/sqlprogrammability/archive/2007/01/17/10-0-plan-cache-flush.aspx

http://www.sqlservercentral.com/Forums/Topic377116-146-1.aspx#bm380502

Do any of the items in the MSDN blog play a factor in your environment?  Most common one would be the AUTO_CLOSE ON.  Let us know, thanks.