when posting articles from our WordPress blog tvnoir.de on the Facebook timeline preview images are not shown anymore.
The error occurred about three weeks ago, we can't determine the exact point of time. Now even when we're reposting older posts, which had been published on Facebook before with working preview image, the preview image specified in the open graph meta tags is not appearing in the Facebook post. When the article is shared on Google+ the correct preview image is shown.
I tried various ways to determine the cause for this. The Facebook Debugger shows the message: "Object at URL 'http://tvnoir.de/premiere-alin-coen-band/' of type 'article' is invalid because the given value '' for property 'og:image:url' could not be parsed as type 'url'." (https://developers.facebook.com/tools/debug/og/object?q=http%3A%2F%2Ftvnoir.de%2Fpremiere-alin-coen-band%2F). But I can't see the reason for this error, the source code seems to be fine.
So if somebody has a hint for me where to problem could originate I would absolutely glad!
Thank you in advance