Movie and TV shows posters are all screwed up

[prefatlaval]

New user here. I'm running Jellyfin server on a Terramaster NAS. At first all was running fine, I had all my movies showing with it's proper poster. Same for TV shows. Then, I did something (no idea what) and now, either I get a blue box with the pictogram (TV of film board) in the middle, or I get an image that has been taken from the movie. If I click on one of the movies, I get the proper synopsis (year, duration, genre, director, actors) but where the actor pictures should be, I get a dark gray box with a pictogram of a person in the middle.

When I go the the files on the NAS, I can see in every folder (one for each film), that there is a "movie.nfo" file and with some of the movies, there is also a "folder.jpg" file, containing that snapshot from one scene of the movie.

In the .nfo file, there is a line with "<poster>/path to the film.../folder.jpg</poster>" or one with "<poster>/Users/my-usern-ame/Library/Application Support/jellyfin/metadata/library/7a/7a1dfff5b589f9e1288421c824bbdcef/poster.jpg</poster>.

In the scanning options for the libraries, I have:
Prefer embedded titles over filename -> On

Metadata downloaders (Movies)
TheMovieDb -> On
The Open Movie Database -> On

Metadata savers
Nfo -> On

Image fetchers (Movies)
TheMovieDb -> On
The Open Movie Database -> On
Embedded Image Extractor -> Off
Screen Grabber -> Off

Fetcher Settings
Fetch Images
Primary -> On 
Logo -> On
Thumb -> On

Did I set the wrong settings in the scanning options? Is there a way to reverse all this and re-scan to get the proper posters?

Thanks for any help.