Same problem here, Also disabled all other plugins and that didn’t make any difference.
The PUT request is succesful but it doesn’t save the (new) fields and there are no error’s visible in the console.
When creating a new form it only succeeds on a few (3 or 4) default items, like one header, a selectbox and a single text line. Any more or custom slug’s and it won’t save.
Also after adding (not custom name/slugs) saving the initial form, then adding a few form fields it won’t save the new fields.
The PUT does send the new fields but the response doesn’t contain the new fields.
Even on a new install and only this form plugin editing forms is impossible. After creating a form with 5 elements and whithout editing the slugs, saving and editing again the new elements aren’t added.
Form version 6.4.3
Wordpress version 4.1.1