I just tried it on my mobile browser and came across this issue. The cerificate is issued by GoDaddy. According to GoDaddy this problem has something to do with an intermediate certificate not being installed. I don't know what that means or if it's actually relevant.
Looks like auth.giantbomb.com isn't sending the intermediate certificate.
The certificate chain (as reported by Chrome on my PC) looks like this: auth.giantbomb.com -> Go Daddy Secure Certification Authority -> Go Daddy Class 2 Certification Authority
However, auth.giantbomb.com sends only the first certificate. While chrome on android knows about the last certificate, it doesn't seem willing to go and find the intermediate certificate on its own so it can't validate the first certificate.
This is in contrast to https://www.godaddy.com which sends both the first and intermediate certificates in its response.
This page explains the problem in more detail (as well as why it's a problem with the site rather than the accessing device): https://code.google.com/p/android/issues/detail?id=6657