I cannot execute a backup maintenance plan. I get a 15404 error,
stating that it "could not get information on Windows NT group/user
'DOMAIN\user'.

SQL Server Agent is set to use Local System Account logon permissions.
SQL Server 2005 is running on a Windows 2008 server which used to
belong to an AD domain. I changed the server to belong to a workgroup
instead.

But in executing the backup maintenance plan, it is still trying to
use the DOMAIN/user credentials.

What do I need to do to fix this?

Thanks, Lars