Some time ago we purchased a SSL certificate from Comodo for a client to use on their site. When we first installed it everything worked smoothly.
Recently however, we noticed an issue where we were getting a SSL warning page with the error:
The certificate is not trusted because the issuer certificate is unknown.
Error code: sec_error_unknown_issuer
After lots of digging and reading I figured out that the “intermediate CA” had been changed. It was a simple matter of re-downloading the SSL from Comodo’s admin interface and installing the new client_domain.ca-bundle file.
It would have been nice if Comodo had emailed me to tell me the certificate had changed/updated.
For instructions on how to install a SSL (in case you’ve stumbled upon this link and are looking for that) use these instructions as a guide (no promises it’s up to date).