My clients are getting "There is a problem with the proxy server's security certificate. Outlook is unable to connect to the proxy server server.sub.domain.com. (Error Code 80000000)." This is only happening from outside of the network. I have a SAN certificate with the server name above, as well as with the outside name mail.domain.com (external), autodiscover.domain.com (external), and autodiscover.sub.domain.com (internal). The error can be dismissed, and Outlook works normally afterwards.
Remote connectivity analyzer shows no errors for Outlook Anywhere or EWS. I have removed all other certificates from my Exchange server. I have verified that the internal/external URLs are correct for Outlook Anywhere, OWA, ActiveSync, AutoDiscover, etc.
Any ideas of what may be going on?