The communication to the proxy is https. As well as the communication between the proxy and Zimbra.
I use the same certifiaces for apache and Zimbra.
my httpd.conf looks like:
RequestHeader set Front-End-Https On
Allow from all
SetEnv force-proxy-request-1.0 1
SetEnv proxy-nokeepalive 1
Beside this, I have many other 'Location' where I use a proxy for other services on different servers. They all work without any problem. They all use https as well.
So far, I tried all kind of combinations in the config-file...