I get the same error, but only if a field is left empty when submitting; I’m adding this both to let people getting this error know it is not simply because the php mail function may not be enabled, and I’d like to know if there is a way to resolve this.
The form setup is all text fields, no special validation, not required, with placeholder text. A workaround is making all of the fields required, but I would really prefer not doing so.