Browser error when connecting to Zarafa webapp URL

Hey guys
Recently I started getting the following error in the browser when trying to access my mail server via 'webapp' or 'webaccess' from outside my network

Secure Connection Failed
An error occurred during a connection to mailservername. Peer received a valid certificate, but access was denied. (Error code: ssl_error_access_denied_alert)
The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.

I've tried different browsers without success. Activesync continues to work as before, as does Outlook.
I'm running ClearOS 6.latest with updates applied.
Any pointers to fault finding or a resolution muchly appreciated.