Not sure if you’re aware of the discussion at advancedcustomfields.com, and http://wordpress.org/support/topic/interferes-with-acf-relationship-field but there is a conflict between Events Calendar 3.0 and Advanced Custom Fields 4.1.8 – Relationships.
Right now it looks as though some of the folks ACF are looking into it but (as of the last post) getting nowhere. I’ve been checking into this problems myself and have been able to track it down to the point where relationship.php attempts to get the relationships using get_posts (line 695). It seems that if the array of post types includes ‘tribe_events’, get_posts returns nothing, if it is unset however, the call returns the expected result.
This suggests to me that the problem might be on TEC’s end. I’ve tried tracking it down further but so far I have failed. Perhaps someone at Tribe can use this information to expedite a solution, or at least determine which plugin is at fault.