Ok, i think we solved the problem. What is didn't mention is that we are using NTLM authentication
When using NLTM Authentication by jCIFS in combination with Tersus and Tomcat we saw sporadic NullPointerExceptions in Tersus/Tomcat. This was due to IE explorer trying to negotiate a NTLMSSP on every POST back to the server, called NTLM pre-authorization When issueing much requests in a small period of time (i.e. fast clicking/navigation by the user) this problem occured. The behaviour of IE in negotiating NTLMSSP on every post can be disabled by setting a registry key. Disabling the NTLM pre-authorization fixed our problem See HTTP POST and Protecting Sub-Content http://jcifs.samba.org/src/docs/ntlmhttpauth.html You cannot post data to a non-NTLM-authenticated Web site http://support.microsoft.com/kb/251404 Background on similar Ruby problem http://lists.rubyonrails.org/pipermail/rails/2006-March/027283.html
Thanks for your help.
For best results, use the Firefox browser..