@morrow without logs this will be hard to track down. can you ask your administrator to open up a support case for this?
Officially we support Mozilla Firefox 3.6 up to the latest version, and Internet Explorer version 6* to 8. We recommend Firefox as it is more secure and performs better.
Although not officially supported most modern browsers are known to simply work with the Zarafa WebAccess. Starting from ZCP-7.0 we plan to add Google Chrome and Apple Safari to the list of officially supported browsers.
*) Our next generation WebAccess, of which a beta will be part of ZCP-7.0.0, will not support IE6.