I think I have found the origin of the problem. There are differences between the pictures coming from Tmdb when all was good and now:
The good old ones of cover.jpg and folder.jpg were in 300 dpi resolution, they are now in 72 dpi.
The good old one of fanart.jpg had a jfif information, it has no jfif information now.
I don't know how to change that with a PC, but it is quite easy with Graphic Converter with a Mac. The pictures are to be modified and saved in jpg progressive with jfif information.
After that you can delete the bad about.jpg in the movie folder, and add the folder including the movie folder in jukebox+ : a good about.jpg will be generated in the jukebox+ and you can copy it to thr original movie folder. And all is good so

It would be better if Tmdb or Mede8er do something

Excuse my poor english...