Is there a built in limit to the number of email accounts allowed?

Done a fresh install of Zimbra Desktop 2 beta 3 (on Win XP).

On adding 15th email account, the account setup window gives the following error:-

HTTP ERROR 500

Problem accessing /zimbra/desktop/accsetup.jsp. Reason:

com.zimbra.common.soap.SoapFaultException: no valid authtoken present
ExceptionId:com.zimbra.common.service.ServiceExcep tion: no valid authtoken present
ExceptionId:btpool0-12:1276344989156:a9f449fbf1201046
Code:service.AUTH_REQUIRED

I have have this problem since trying all of the Zimbra Desktop 2 betas and never used to have any problems running 20+ email accounts on version 1.x.

Has anyone else experienced this problem? I'm back to Thunderbird until this is fixed.