 <p>It can also be caused by <a href="https://bugs.eclipse.org/bugs/show_bug.cgi?id=475368" rel="nofollow">this</a> bug, if you're having Eclipse 4.5/4.6, an Eclipse Xtext plugin version older than v2.9.0, and a particular workspace configuration.</p> <p>The workaround would be to create a new workspace and import the existing projects.</p>