When I run my grails app locally under Tomcat, I get no errors. When I deploy my WAR to my remote web server, I get this exception when I try to make an AJAX request that uses the HTTPBuilder.
Are you running an IDE application like STS? if so, it is possible that the Jars for the HTTPBuilder are in your build / run path but are not in your lib directory for the Grails application, or listed as a "production" dependency.