Okey this is now solved. As I suspected this was a problem with WP-Engine. Thanks to Elliot who has contacted their support in order to get an answer for this issue. Kind of weird that the support I talked to didn’t know about this, but:
Thanks for the update! We recently made a change to one of our mu-plugins to force backend requests loaded over the WP Engine URL to load over HTTPS. This includes the /wp-admin/admin-ajax.php POST requests handling this form data. This is causing an issue with the form population on the page below, and I’ve gone ahead and disabled this functionality on our end to correct the issue.
The support could fix this for my certain install. But at the moment there is no “global” fix.
