When this plugin is activated the new user registration is being blocked. New users can click on the registration email link and it appears that they are activating their account but in the User manager, they are still shown as NOT ENABLED and can not log in. After deactivating and uninstalling the plugin, email activation works properly. It also breaks my template with SEF turned on. This would be a good plugin but too many bugs.

This site has SEF turned on and has none of the issues you mention. It can only be caused by the interaction of another component or something odd in your server config.

Do you use any components/plugins that may affect registration that aren't listed as supported on the Registration Validator page? If so, please list them here and I'll see if anything crops up.