First off, check your default domain controller policy to sort out your incorrect SID. It’s most probably power users in the windows security settings.
Static IPs won’t give you a problem unless you run DHCP as well and hand out the same IPs. Make sure subnet masks are correct.
Your problem does look like an ISA server issue. I seem to remember our installation of ISA 2000 doing the same thing. Make sure you’ve told it your internal address ranges and make sure you’ve patched it. Have you looked at the routing tables and LAT?
Have you checked your DNS as the error you’re coming up with is referring to the AD PDC, I believe. This may be causing your routing issues.