I wrongly reported it as a Devise bug, when is actually a aws-s3 bug:
heartcombo/devise#3220
Looks like aws-s3 is messing with ActionView::Base.default_form_builder, that instead of returning ActionView::Helpers::FormBuilder is returning nil - this can be replicated by justing generating a scaffold in a fresh app with aws-s3, even without Devise.
Looks like aws-s3 is overriding the cattr_accessor method from ActiveSupport and breaking some Rails internals. I think this should be reported back to the aws-s3 repo and fixed and their end.
I wrongly reported it as a Devise bug, when is actually a aws-s3 bug:
heartcombo/devise#3220