_After installing Pi-hole, you can open the web interface. _
After entering the Pi-hole IP adress while trying to connect to the Pi-hole web interface, the web browser says that it refused to connect.
The operating system is Ubuntu MATE. Also, I am a complete linux novice, don't really know how to navigate in this OS
Your debug log shows your Pi-hole has issues with its network connectivity, sporting multiple IPv4 addresses for the same interface, but the main issue is dnsmasq being installed:
*** [ DIAGNOSING ]: Ports in use
[53] is in use by dnsmasq
[53] is in use by dnsmasq (https://docs.pi-hole.net/main/prerequisites/#ports)
dnsmasq is preventing Pi-hole from accessing port 53, and thus to operate correctly.
See if disabling and/or uninstalling dnsmasq would improve your situation:
sudo service dnsmasq stop
sudo service dnsmasq disable
You may have to reconfigure Pi-hole afterwards:
pihole -r