Hi Sebastian, Thanks for yoru reply. 

Yes we've done this and these settings are correct.

We have now narrowed it down to the Firewall - the file manager tree works fine when connected to the site on the internal network, but not from outside on an internet connection through the firewall.

Any ideas on what might be being blocked?

Tim

Hi guys,

We think we might of found a resolution for the problem!!

Our HTTP proxy on our WatchGuard was performing a "ProxyStrip: HTTP Header match" which was stripping out the header="X-AspNet-Version: 2.0.50727\x0d\x0a", header="__DNNCAPISCSDI: \x0d\x0a" and one more but I failed to copy it.

 For testing purposes I added X-AspNet-Version: * and __DNN* to the HTTP Proxy Header Fields and the drop down boxes now appear to be working.

Hope this helps

Steve