They are not there anymore (they are there but very small). They disappeared after I made the CSS changes I was asked for. I will remove the CSS so you can have a look for a good solution?
Update: I put the CSS in the Custom CSS of the template. Now I have put it in the custom CSS of the thumb slider where the problem occurs. When I leave it out I have the thumbnails again, bur not centered. So I guess the provided CSS is not good for centering the thumbnails.
div#carousel{
display: flex;
justify-content: center;
align-items: center;
}