Example URL: https://gabrieleromanato.com/2017/03/jquery-disabilitare-il-copia-e-incolla-delle-password/
Tests with cUrl: 1) SSL ok 2) 0 HTTP errors 3) robots.txt does allow Twitterbot 4) no server-side limitations
Image size: less than 1Mb, tested with different formats, same results.
Site’s hosted on my VPS on Digital Ocean.
Update: it works now. You can close this thread.