Add comment function not working on proxy with https web server

I know the problem with apache proxy and https is known, but this one is a more specific one. My setup is apache web server, https only, with mod_proxy and standard configuration for proxy.

Everything works fine, except "Add comment" (the AJAX call), it seems to call http:// instead of https://. Everything link on the page is pointing to https://, so I believe my configuration is correct. I haven't notice this problem in any other part of xwiki, I can navigate to every page, upload files, etc. I just can't add a comment without hitting http:// (I confirmed this in the apache logs).