When I visit your example page on my iMac in Safari I see this…
And in Chrome and Firefox I see this…
The reason you are seeing placeholder images for some posts, or no image at all, is because those posts do not have a featured image set.
On your example page the “wie alles begann” post has a featured image set…
But the other four posts don’t. For example, this is what the “na-nu-1″ post looks like…
As to why the placeholder image is not being displayed on all devices and in all browsers, I can’t really say, I’m no expert with the WPBakery Page Builder plugin. It’s maybe a bug in the plugin?