There is a ZUL file:

<?forward uri="zk/foo.zul"?>
<zk></zk>

On Jetty 6.1.26 (with RJR) will forward correctly, but on Tomcat 7.0.x will occur HTTP 404.

Another case, the foo.zul was put in foo folder:

<?forward uri="foo/foo.zul"?>
<zk></zk>

The Jetty 6.1.26 / Tomcat 7.0.x will work fine.

Reference: http://www.zkoss.org/forum/listComment/20708