I have gotten this error before and to fix it, I created a new external list and new form. But when going back in and editing the new form, it doesn't even publish and keeps giving me this error. I tried creating a new external list and form again, but it still keeps giving me the same error. I also tried the recommended sharepoint solution to add the Desktop feature, but my computer already has it installed. I am not locked out of my account either.
How did you edit the new form?
I can publish the form successfully via SharePoint Designer->Edit external list->Design Forms in InfoPath under Actions tab->Item.