I got the problem on Linux Mint 18.3 (with four different browsers), with Linux Mint 19 (five different browsers) and on Ubuntu 18.04 (three different browsers).
On the Ubuntu box, I changed the /etc/resolv.conf from the Pihole raspberry pi to 1.1.1.1 directly. And it worked just fine.
Thus, there is something not working between https://ici.tou.tv and Pihole.