A good way of finding what is causing this is to look at the Active Sessions (Management Console > Statistics > Sessions > Errored sessions). In this case we can see Google talk is causing the error:
talk.google.com:443 - - - REQMOD: inactive RESPMOD: inactive 20 sec 176 0 n/a - - - P BM (D) Explicit HTTP SSL SSL(error) : "Missing SSL server certificate" 0 sec
Google talk will by default try to connect over port 443. It is an application. The site does not provide a server certificate, which is required for the SSL proxy to work.
Any site which does not provide a certificate will result in this error. To resolve the error, bypass the SSL proxy for this site.