TaxonomyFieldControl You must specify a valid field value

I have a custom Manage Metadata field which is hooked up on a TermStore which has an Open Submission Policy and the field itself has Allow multiple value + Fill-in choices enabled (Checked).

Yet when I try to create or edit an item and fill-in a new term which I want in the termset, I get the error You must specify a valid field value.

Any ideas?