Hi Takayuki,
I have done the same as @eddycom - and I am running all of the same plugins on another Headway site - and it works fine. I get the "spinning arrow" as well as the inablility to add new forms, when I see the "Enhanced Custom Fields configuration error: Cannot load -- unknown POST ID" error message.
I am not using ANY enhance custom fields plugin - and even if I had been, I disabled them all to no avail.
-Brian