Hi, @smurf (Luca DG),
Besides the good suggestions that @tkn (thom) has already given you here in this topic, I've found the following similar Topic / Conversation ("Can't get PDF Thumbnails") here in the "Ubuntu MATE Community", started by @koana33 (Dakinu) about 2 years ago (on June 2020):
Reading that topic, it seems that one other thing you might still check is to ensure that the "Document Viewer" in the "Office" tab in "Preferred Applications" is set to "Atril Document Viewer" as @olek (Alexandre) shows at the end of the following message in that other Topic:
I hope this helps in some way.