I’ve looked at your site, and I might be able to guess the problem. Did you upload all, or a great many images in bulk, perhaps via FTP?
That is OK, but when you do thumbnails are not immediately created. It would take far too much server resource/ So they are created as each image is viewed. The 500 error is basically telling that, at that instant, the thumbnail doesn’t exist.
If you click on one of those images that is not displayed you will see the full size image correctly. Then close it, and refresh the whole browser page. You will now see the thumbnail appear.
There is no quick way to do this, but you can go scroll through all the images in Administration > Media > Manage media. The ones with no thumbnail are easy to spot. So just open, then close it and move on to the next. Eventually you will catch them all.