I was hit by Chrome Mixed content Autoupgrade experiment that upgrades http request made from content loaded with https to https-requests. Since TSO uses mixed content I couldn't load the game. The problem was w02bb01.thesettlersonline.com/crossdomain.xml that isn't available via https.
I managed to contact the Chromium developer who is responsible of that experiment and he gave me a workaround that disables the experiment. Adding --disable-features='AutoupgradeMixedContent' to the Chrome command disables the experiment and allows TSO to load.
I hope if someone else if hit by this problem can find this post and the workaroud.