This is an error that I’m seeing more often with Grails 2.2.1. It seems a bug with the Spring Reload Agent.
There’s an open JIRA ticket for this, but have an workaround. If you use linux, delete ~/.grails/<version>/.slcache. If you use Windows, the location is <workspace>\.metadata\.plugins\org.grails.ide.eclipse.core\2.2.1\.slcache.