A "view source" on the HTML may also reveal a java stack strace which can be useful for debugging.
This error means the JSP tag libraries couldn't talk to your server. Take a look at:
Code:
/opt/zimbra/tomcat/webapps/zimbra/WEB-INF/web.xml
In particular, this setting should point to your server:
Code:
<context-param>
<param-name>zimbra.soap.url</param-name>
<param-value>http://localhost/service/soap</param-value>
</context-param>