thumbnails won't display

hello dear community,

Since today i have beeen having a problem i don’t seem able to solve.
I have currently 3 galleries. When i click on each of them only 3 thumbnails are displayed, although all the pictures are loaded; I have not done any change to the administration of the site other than uploading more pictures;

Can anybody help please?

My website is: https://www.panhdr.com (Lyon en images)

WordPress up to date: Yes
NextGEN up to date: Yes

https://www.ads-software.com/extend/plugins/nextgen-gallery/