When installing this plugin and adding the Upcoming widget, I encountered the following error when saving changes:
Fatal error: Call to undefined function get_upcoming_events() in ...\wp-content\plugins\upcoming\upcoming.php on line 236
I searched and could not find the get_upcoming_events() function anywhere.
So I changed line 236 from:
$events = get_upcoming_events($options['userid']);
$events = get_upcoming_events_for_user($options['userid']);
The widgets page now loads without error and the events display as expected.
Hope this helps someone, but also as a note to Yoost to investigate further when he has the time :)