When editing a post, the option in the poll tab:
"Options per user"
Apparently is set as 0 because $post_data['poll_max_options'] is "0" (transforming to int it gives: 0).
The HTML input element where that goes to is of @type=number and has @min=1 and @max=99 so the browser throws an error when I try to submit that form.
As generic as the browser needs to be, the error it throws is not obvious because it is unable to point to the right input (as it is hidden) so it took me a while to find the real problem.