500 Service Unavailable - ISA 2006 and SharePoint 2007

Using SharePoint 2007 and ISA 2006 I came across an issue where external users of a site were recieivng the error "500 Service Unavailable" - even though the site was available and working. After some digging around I found the following resolved my issue 


I was required to switch off "verify normalisation" on ISA - it seems its an incompatibly between ISA 2006 and SharePoint 2007