This exact same issue (single events not showing) just happened to me.
I fixed it by changing the magic number ’10’ in add_action( 'init', array( $this, 'init'), 10 ); in lib/the-events-calendar.class.php to a lower one, and behold, single events started to work again.
With 10 or higher numbers, wp_query was looking for an attachment, just as syrup core outlined above.
Obviously this fix is not ideal: is there a way to change the fixed magic number 10 to “as low as needed” in code of “The Events Calendar”? Would certainly be easier (at least for me 😉 than having to go through all the other plugins and find out what the culprit is. It would also make “The Events Calendar” more robust, avoiding the weird queries with attachment types…
