p.dirac on "WP 4.0 - Plugin could not be activated because it triggered a fatal error"

I'm trying to upgrade to wordpress 4.0, and get this error,
"Plugin could not be activated because it triggered a fatal error", when activating mailpress 5.4. All other plugins are deactivated, and the theme is set to twentyfourteen. The mailpress 5.4 plugin works fine in wordpress 3.9.

There is no clue what the fatal error is. In wp-config.php, I have set:
define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', true);
ini_set( 'display_errors', 'On');
ini_set( 'log_errors', 'On');

Is there anyway to display the fatal error from a plugin activation?