Twiter Cards stopped working... w/o syntax errors


Our Twitter Cards stopped working all of a sudden, around February 24th.

The weird thing: The Debug Tool on gives two Errors when I feed with a Media URL, but works fine if I paste in the page’s source code.

So or result into:
• Missing Tag: twitter:title
• Missing Tag: twitter:description

However, these tags are present – and if I paste the page’s source into the debug tool’s textarea, it validates fine and also shows the Twitter Card Preview.

What am I doing wrong?