Comment 6 for bug 1817903

Ok I have found what the problem is, I think; I suspect that you have resolvconf installed on your system, which is taking over the /etc/resolv.conf file, and I think that resolvconf is pulling the edns0 option, but also including external nameserver(s), which is an incorrect configuration, since the edns0 option should only be used when talking to the local stub resolver, because we don't know if external dns server(s) support edns.

The resolvconf package is not installed by default for Bionic (though I am not sure if Mint installs it by default for some reason). Is this system upgraded from a previous release, or did you install resolvconf manually? Unless you specifically need it, can you try unistalling resolvconf and rebooting to see if that fixes the problem for you?

$ sudo apt remove resolvconf
$ sudo reboot

And also please do paste the contents of /etc/resolv.conf here, especially if removing resolvconf doesn't help.