Some Gallery Thumbnails do not show

Hello,

On our site, we are using NextGen Gallery 2.0.40.
We have created various albums (1 per year) which are composed of multiples galleries.
For some of our older Albums, the thumbnails of the galleries do not show while everyting is working fine for the most recent Albums.

Here is the link to a recent Album page with the galleries thumbnails : http://www.t21.ch/photos/galerie-photos-2013/
Here is the link to an older Album page with the missing thumbnails for the galleries : http://www.t21.ch/photos/galerie-photos-2009/

All Albums are using the “NextGEn Basic Thumbnails” for displaying the galleries without any template.
I tried to change the preview image of the galleries or override the thumbnail settings in the Album settings. Without any success.

So, would anyone have some sugegstions to fix this?

Thanks in advance
BR
Alexandre

https://wordpress.org/plugins/nextgen-gallery/