Hi Henrik,
After a fair amount of research on our end, we cannot see why the DEVONWAY\453432-DB1$ login is being used in SQL Server. As far as we are concerned, we do not want the DEVONWAY\453432-DB1$ login to execute the stored procedure. One thing we are wondering is whether VisualCron is somehow using impersonation when you convert the SQL Task to SQL Command "EXECUTE <stored_procedure>".
Also, this error shows up only some times, so we are not sure what would cause it to impersonate only some of the times. We are on SQL Server 2005 SP4.