Download WebSite 3.1.13.0 (which contains the service pack mentioned above): http://www.deerfield.com/download/website/
Release Notes: http://www.deerfield.com/support/website/releasenotes/
Bob Denny, author of Website, also mentioned this workaround to the problem: "Don't use any active documents with file extensions longer than 3 characters. Remove the mapping for .html-ssi or change it to .ssi and rename all your .html-ssi documents. Fix all the links to same. I know it's ugly, but upgrading would solve the problem."