I use a permalink structure of mydomain.com/articles/%postname%/ which means that events were showing up as mydomain.com/articles/event/%postname%/ which is wrong taxonomically speaking.
Using the Custom Post Type Permalinks plugin wasn't letting me change that because the ai1ec_event CPT has with_front hardcoded to true.
For now I've fixed this for myself by going into all-in-one-event-calendar/lib/post/custom-type.php and changing
$rewrite = array( 'slug' => Ai1ec_I18n::__( 'event' ));
$rewrite = array( 'slug' => Ai1ec_I18n::__( 'event' ), 'with_front' => false);
But this will break whenever I upgrade the plugin.
It would be great if you would let us toggle with_front in the plugin settings.