Seems like reading through these forums this is a common problem, though I have yet to see any tips on resolution.
When first setting up two sites a month ago I got my twitter cards approved & the validator worked. Now when I try any url from either site in the validator it returns “unable to render card preview”, with ‘Error: Internal Server Error’ in the log section below. My posts - likewise - are not being ‘twitter carded’. No summary section, etc.
Any clue? This is frustrating. Been trying to get the validator to work for 2 days now.