I have two Windows 2008 x86 servers running IIS 7.0, one site on each server; both sites are SSL-enabled, using DoD-issued certificates. Both sites are accessible via https over port 443, but fail the moment Client Certificates are set to Require or Accept. IIS log records error 500.0.64 but nothing else. I have several Windows 2008 IIS 7 x64 servers that require client certificates and they are working as expected; it's just the two x86 servers that are being problematic.