Note to self: need to install SP1 for ISA 2004. From the download page:

ISA Server 2004 Standard Edition SP1 can be installed directly on computers running ISA Server 2004 Standard edition, and includes:

• All software updates issued since ISA Server was released to manufacturing.
• Fixes for common issues reported by customers through Microsoft Product Support Services (PSS).
• Enhanced stability of the ISA Server services and administration tool in a number of scenarios.

Also check out the readme.