In the latest version, I noticed a 500 error on the Widgets page when adding Upcoming Events widget to my sidebar. The widget appeared to load and save fine, but I noticed the error in debugging (Safari). There was a double forward slash in the path to the widget.css file the widget was trying to include.
The extra forward slash is being added in the admin_enqueue_scripts() function found in the following file: all-in-one-event-calendar/app/helper/class-ai1ec-app-helper.php.
If you are also getting this error, try the following:
On approximately line number 902 change this line......
$ai1ec_view_helper->admin_enqueue_style( 'ai1ec-widget', '/widget.css', array(), AI1EC_VERSION );
to this(removing forward slash in front of widget.css).......
$ai1ec_view_helper->admin_enqueue_style( 'ai1ec-widget', 'widget.css', array(), AI1EC_VERSION );
Hope this helps someone and can be fixed in an upcoming version.