@Mr Papa We are seeing the same symptoms, but are not running our browser behind a proxy. These issues appear to have started with an update about 3 weeks ago.
After the login button is clicked and the /wp-login page fails, the /wp-login page works when requested by Refreshing the browser tab.
Server log reveals multiple instances of what appears to be the same error. Any insight you can provide would be very helpful!
[Tue Aug 30 10:12:05.044939 2016] [:error] [pid 17041] [client 73.12.102.1:16530] WordPress database error Not unique table/alias: ‘wp_usermeta’ for query SELECT SQL_CALC_FOUND_ROWS wp_users.* FROM wp_users INNER JOIN wp_usermeta ON ( wp_users.ID = wp_usermeta.user_id ) INNER JOIN wp_usermeta wp_usermeta ON ( wp_users.ID = wp_usermeta.user_id ) WHERE 1=1 AND (
( wp_usermeta.meta_key = ‘pw_user_status’ AND wp_usermeta.meta_value = ‘denied’ )
) AND ( (wp_usermeta.meta_key = ‘pw_user_status’ AND CAST(wp_usermeta.meta_value AS CHAR) = ‘pending’) ) ORDER BY user_login ASC /* From [exlog-global.com/wp-admin/users.php?s&pw-status-query-submit=Filter&new_user_approve_filter=pending&paged=1] in [/nas/content/live/wfaexlgl/wp-content/plugins/new-user-approve/new-user-approve.php:309] */ made by require_once(‘wp-admin/admin.php’), require(‘wp-admin/menu.php’), require_once(‘wp-admin/includes/menu.php’), do_action(‘admin_menu’), call_user_func_array, pw_new_user_approve_user_list->pending_users_bubble, pw_new_user_approve->get_user_statuses, pw_new_user_approve->_get_user_statuses, WP_User_Query->__construct, WP_User_Query->query, referer: http://exlog-global.com/wp-login.php?redirect_to=http%3A%2F%2Fexlog-global.com%2Fwp-admin%2Fusers.php%3Fs%26pw-status-query-submit%3DFilter%26new_user_approve_filter%3Dpending%26paged%3D1&reauth=1