I'll try my best to explain this issue

I just upgraded to Zimbra 5.0 Beta 3.

Everything works perfect when connecting from our internal network.

However, when accessing from Internet via our reverse proxy server and using the "Advanced" version, when I click the log in button all that happens is a blank page appears and just refreshes itself every second or so.

I tried selecting the " basic" HTML version and that worked fine. I was able to see my e-mail.

Also, the advanced version worked fine through our proxy server in Zimbra 4.5.x

Where would I look to try and figure out what the problem could be ?