Log full of javax.net.ssl.SSLException errors

[Daniele]

Hi,
could someone help me understanding what this error means and how to get rid of it?
Every day my log is full of messages like this:

IO error: javax.net.ssl.SSLException: Connection has been shutdown: javax.net.ssl.SSLHandshakeException: Received fatal alert: certificate_unknown in processing a request from /192.168.1.12:8443 / sun.security.ssl.SSLSocketImpl

I read other posts telling is was related to some script under "HTML additional headers", but that section is completely empty in my case.
It's probably something simple about https certificates, but I don't have any experience on this kind of things...

Thanks
Daniele