Hi Rocco,
Yes: you’re right about the ccs in my child theme causing that. In fact that was my main suspect. Yesterday finally I managed to fix it tweaking the css until I finally hit the right keys.
What surprises me, and what made me post my problem here, is the fact that supposedly the new versions of the theme were child theme safe, but in my case this has not been the case, as my previous css worked perfectly in version 3.3.12, but not with the next versions.
Many thanks for your help.