Preview Thumbnails on New Tab page empty only for certain URLs

When I open a new tab, there are three certain preview thumbnails which are always empty, and there is no way I can make them show up again. They did work in the beginning, but at some time the three previews vanished and never came back. I have already tried to empy the cache and reload the sites as suggested here, but the thumbs are still empty. It's always the same three URLs where this happens. I have also cleared the history, opened the sites again and dragged the items from the history menu into a thumbnail - same result.

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