I must consume a PHP webservice which has a SSL certificate. My .net 3.5 Class library references the webservice with \'Add Service references\' in Visualstudio 2010 (WCF ri
Yes an Untrusted certificate can cause this. Look at the certificate path for the webservice by opening the websservice in a browser and use the browser tools to look at the certificate path. You may need to install one or more intermediate certificates onto the computer calling the webservice. In the browser you may see "Certificate errors" with an option to "Install Certificate" when you investigate further - this could be the certificate you missing.
My particular problem was a Geotrust Geotrust DV SSL CA intermediate certificate missing following an upgrade to their root server in July 2010 https://knowledge.geotrust.com/support/knowledge-base/index?page=content&id=AR1422
(2020 update deadlink preserved here: https://web.archive.org/web/20140724085537/https://knowledge.geotrust.com/support/knowledge-base/index?page=content&id=AR1422 )