Adding new users causes permissions error

With the Remove Email Verification Plugin on our multisite network, when adding a new user to the site we are getting a "You do not have sufficient permissions to access this page." error every time.

If you then click the back button on the browser and click on the dashboard link (or any other admin link) you see that the new user is logged in.

This is obviously why the permissions error is showing as new site users (with subscriber access) can't access the add user admin page.

The question though is why just adding a new user triggers that user to be logged in? The new user should just be added to the site and the admin user remain logged in.

This is the only active Plugin on the site, we have disabled all other to rule out Plugin conflicts. We are using version 2.1.1 of the Plugin and running WP 3.5.1.

Please advise. This is a serious problem for us.