Thumbnails do not show anymore / automatically deleted from server?

Dear final tiles Support Team,

recently i noticed that older galleries on our website do not show thumbnails anymore. The gallery seems to be there because you can click into the white area where the thumbnails should show. Also the lightbox opens and shows the correct images in full size when clicking.

We already found out that the generated thumbnails are not on the server anymore. Here is an example:
https://tapethatcollective.com/projects/adidas-the-matchroom/
The big image is there: https://tapethatcollective.com/wp-content/uploads/2016/11/Matchroom_adidas_tapethat_Tapeart_Tape_art_11-1024×683.jpg

… but the thumbnails seems to be gone.

Any ideas how i can solve this problem without having to recreate all older galleries?

Thanks a lot.