I am also having this issue. On attempting to validate my website for Twitter cards using the card validator I am getting the message ‘Unable to render card preview’ with the following error log:
INFO: Page fetched successfully
WARN: No metatags found
I have set up my meta tags as follows in this example:
<!-- Twitter -->
<meta name="twitter:card" content="summary_large_image">
<meta name="twitter:site" content="@TheTwitterSite">
<meta name="twitter:creator" content="@TheTwitterUser">
<meta name="twitter:image:src" content="http://cnd.domain.tld/images/cardimage.jpg">
<meta name="twitter:title" content="The Sites Title">
<meta name="twitter:url" content="http://domain.tld/">
<meta name="twitter:description" content="A useful example description">
I would assume that the above would meet the minimum requirements to load the card data. I also have other OG tags here for sharing on Facebook.
On running a curl -A “Twitterbot/1.0” http://domain.tld I can see the meta tags are included in the generated html.
Could this be having an impact on what I am trying to do, despite the fact that running curl returns what appears to me to be the correct generated html?
Is there a way we can see what html was fetched by Twitter, along the lines of what you can do with Google Webmaster tools and Facebook?