I've noticed that you set a 'pre_user_query' action hook to remove Admins from user queries.
This is really bad as all other plugins trying to list users won't be able to list admins. Those plugins will seem to be working correctly for the admin (because the action hook won't apply to him) but will appear broken to other users, making the problem even more difficult to fix.
Currently my workaround is to stop using your plugin. add_role, get_role et al. are actually good enough for the job.