After an update , thumbnails show up as dots. If the dot is carefully right-clicked upon, you will see that it is actually a malformed image that is picked from non-existent folders /v and /d under the gallery installation folder.
Go to the site admin panel and click on ‘URL Rewrite’ .You should be able to see the vague paths in that section.
Change or disable the options ‘Show Item’ and ‘Download Item’ and the thumbnails should show up again. This is not an issue with the server.