Why doesn't Vkontakte pick up og:image?

Question

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There are pages in the service that can be shared on social networks. Each of them has a photo that is written in og:image. If there was no photo loaded on the page, then a stub is displayed instead. These pages can be easily shared on vk, fb and ok without any problems. But if there is no photo, and there is a stub, then for some reason vk cannot pick up the stub picture. Fb and ok quietly pick up the photo cover.
What could be the problem? I tried to add the same for the picture, and clean the vk cache of pages, but it did not help.

Answer 1

The stub image hangs on another domain, see if that server does not allow the VK bot, there, for example, protection from hotlink. And look in the logs for the visits of the VK robot, it identifies itself like this

Mozilla/5.0 (compatible; vkShare; +http://vk.com/dev/Share)