There is an update available for the event calendar which should include a fix.
Problem seems to be in line 130 of src/Tribe/iCal.php
$this->generate_ical_feed( $wp_query->post, null );
changing this line to
$this->generate_ical_feed( $wp_query->post /*, null*/ );
solves the problem.
Same here (https://www.drachenmond.de/event/ldn2017/).
ICS file ist zero bytes.
Version 4.4.0.1
