Thickbox flickers and is gone (acf_relationship_create)

Dear Sir or Madam,

I only have this sometimes but can reproduce it somehow:

1. Create multiple post types (with editor and title)
2. Activate the “acf_relationship_create” in a ACF relationship panel (other panels may show the same)
3. Click to create a new entity

What now happens is:

1. If the post type has an editor the thickbox / iframe appears, loads for a second and when it’s ready then it disappears.
2. If I remove the editor from the target post type then the thickbox / iframe “stays open” and I can create an entity.

Please help me debugging this. Write on twitter if you want a screen share.