Thanks for your reply (and thanks for Pleco which allows me to read Liu Cixin in chinese !)
Yes, when I last posted I had the 3.2.25 version , and now I have the 3.2.35 and the problem still occurs.
Pop-ups are empty for some dictionnaries (not all of them), it seems random because with each different word, different dictionnaries may "disappear" or not in the pop-up.
I've reset my parameters and it did nothing.