I have that same problem. To solve it and make authorization I change in /wp-content/plugins/wp-strava/includes/WPStrava/Settings.php:648
from
return isset( $value[0]['type'] ) && 'updated' === $value[0]['type'];
to
return isset( $value[0]['type'] ) && 'success' === $value[0]['type'];
and in /wp-content/plugins/wp-strava/includes/WPStrava/Auth.php:55-57 remove
if ( $settings->ids_empty( $input['strava_id'] ) ) {
return array();
}
