Steps to Reproduce:

Create a page that has no content before the first header (==Header1==) (could be happening with different heading levels as well). Page preview popup for this page will only show "..."

From what i gathered, this is happening in TextExtracts extension actually. If i make the API call without "exintro=true" param, it works (showing raw wikitext, though)

MW: 1.31.6 (65e10c6)
Popups: – (f1678d9)
TextExtracts: – (7b00e9a)