Mixed Content error because of a request to adxk.net

I'm developing a small web app (bootstrap + jquery + nginx + api backend) and today I stumped upon a peculiar error in Chrome, on Ubuntu 16.04.

Mixed Content: The page at 'https://localhost/data/' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'http://ift.tt/2crzZK4'. This request has been blocked; the content must be served over HTTPS.

My page serves all content via https and I'm not making this request.

What (the heck) is adxk.net?