We are getting this error too, but our robots.txt file is disallowing access to the directory where images are stored and I specify a link to that directory. On a side note, I’ve noticed that a number of people (including us) use an “@” symbol in the meta tags values. I’ve definitely had trouble with this before in other places, so just thought I’d mention it in case it’s also a problem here. I’ll report back if we get around the robots.txt disallow issue.
Ignore the @ symbol mention here, but we’re still getting this issue despite many changes to robots.txt file. Sometimes all green lights from the validator, but no preview image, sometimes this error message.