I wouldn’t say the site’s running a lot of theme functions — pretty generic stuff in my opinion. And the only plugin operating is your ACF + repeater and a shortcode plugin.
I thought I did try the relationship field and was still getting the error, but as of right now, I cannot remember.
Consider this forum posting solved or moot (for lack of a better term).
Looking forward to v.5, Elliot. Keep up the good work!