Thank you, I will investigate.
@josevalim After your message I did a mix deps.update
which brought Ecto to 2.0.6 and tried again as in original post. This time it worked perfectly.
Unfortunately I didn’t try before the update so I am not sure if it was a problem with the specific version or a case of PEBCAK or something else. But I don’t remember doing something different than the first time.
Sorry for wasting your time for this.