Verify you have the correct Login ID and Password. You can use the Arcserve Backup Server Admin to change the user id and password.
Run a Database base consistency check (run DBCC in SQL ARCserve Manager) on the SQL database.
Verify you have enough connections to your MS SQL server. Every Arcserve Backup job needs a connection. Go to SQL ARCserve Manager Server menu, SQL Server configuration drop-down menu and check the connection.