It appears that Qt caches old challenges resulting in wrong answers given to cloudflare. Clearing the disk cache causes the problem to resolve temporarily.
The reply from the site contain the following headers:
HTTP/1.1 503 Service Temporarily Unavailable
Date: Tue, 23 Aug 2016 09:23:17 GMT
Content-Type: text/html; charset=UTF-8
Set-Cookie: __cfduid=df1fbdc06b91a16fa61664d8adb3beb131471944197; expires=Wed, 23-Aug-17 09:23:17 GMT; path=/; domain=.abusewith.us; HttpOnly
An example of a site where this is easily reproduced:
I've noticed that I can more easily reproduce this when I do the following:
browse to a site such as the one above and go through the challenge.
Clearing the cookies and trying to browse to said page once again.