So I had MIIS 2003 SP 1 reporting to me that the format of my GalSync-Extension.dll is invalid. So I tried recompiling it -- no luck. Same error. The only MSDN article on this indicated that unmanaged code is being passed to the load method.
Through trial and error we found the solution: stop and start the MicrosoftIdentityIntegrationService. If that doesn't work try a reboot.