Checkbox List and Radio List problem in Twenty Seventeen theme

Hi everyone,

I have a site running Twenty Seventeen theme with WordPress 4.8.1 and Ninja Forms 3.1.9 plugin.
Hence, everything up to date.

Weirdly, on my Ninja Form, items in Checkbox Lists and Radio Lists are always displayed on a new line, above the radio button (or checkbox), like this :
__________________________
This is a checkbox list :
[]
ONE
[]
TWO
[]
THREE
__________________________

The single checkbox though doesn’t have this problem as you can see in here :
https://www.latino.ski/test-ninja-forms

This problem occurs only with Twenty Seventeen theme. It disappears if I switch to Twenty Fifteen or Sixteen.

Is there someone with any idea ?
I am desperate.

The page I need help with: [log in to see the link]