Have an installation with no domain configured (only the internal one). Than, go to VM main tab -> permissions sub tab -> add and click OK. Nothing will happen due to a class cast exception on the frontend. This means it is not possible to add an e.g. user role to the admin@internal for some VM (which is mainly useful for testing).
This is an automated message oVirt 3.4.1 has been released: * should fix your issue * should be available at your local mirror within two days. If problems still persist, please make note of it in this bug report.