We use a wildcard certificate issued by CompleteSSL to do alot of our web mail and services hosting. I renewed it a week ago and installed today, to find that the certificate was missing a trust, even though I had installed an intermediate certificate.
After a little bit of poking around, I discovered that CompleteSSL had changed their CA to Comodo, and that they are trusted via AddTrust. Interestingly enough, thats the same trust that was in place before. I looked further into it and the CA was the same (even the thumbprint matched - 02 fa f3 e2 91 43 54 68 60 78 57 69 4d f5 e4 5b 68 85 18 68), but the new certificate was not accepted still (unknown issuer) .
I installed the certificate found here -https://support.comodo.com/index.php?_m=downloads&_a=viewdownload&downloaditemid=87 as a Trusted CA and the certificate was accepted.
I can't really explain that one, but what a pain in the butt.