redirect loop with standard web client as default

[alagendijk]

I have are very strange problem:
When logging in externally I can use both the ajax as the html client without any problems. But when a user has the html client set as default in his preferences and try's to login the browser complains about a redirect loop both with the ajax as the html client.
This problem only occurs when trying to login externally from inside our network everything works fine.
I tried putting ?debug=1 after the url but that doesn't give any useful information, I also can't find anything weird in the zimbra logs.
The network situation when logging in externally is as follows:
ISA server (I guess, not under my control) -> apache reverse proxy -> zimbra server
Anybody any idea what can be wrong?

[alagendijk]

nobody any idea what can cause this problem?