This is what the error log read after trying to edit a post with integrated ticket:
upstream timed out (110: Connection timed out) while reading response header from upstream, referrer: “https://staging.oor.nl/wp-admin/post.php?post=807441&action=edit”
After editing and unsuccesfully saving the post the ticket within the post is gone. Even a ticket that was created and saved succesfully before editing the post or ticket.