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It's issued by StartSSL and is valid on all the browsers I have tried the site on. Can you tell me what browser (and version) are you using?

Intermediate certificate isn't sent by the server, which might be the issue.

Not the person whom you asked, but I'm getting the same issue on up-to-date mobile Chrome on Note 3 standard build android 4.3

Surprisingly also getting the issue is the Samsung browser on the same android 4.3 phone.