Trying to add a new PPA, using the (already) good-old "add-apt-repository" just hangs and waits forever. Nothing happens. Probably timeout of some kind.
I run into this on every new installation since 18.04, and I keep forgetting what it was. Shame on me...
[SOLUTION]
APT is trying IPv6, but won't get anywhere.
Thanks to Zach Adams fix suggestion (2015), I now now a better way than disabling IPv6 for APT:
1) Open /etc/gai.conf
Make a backup of it before. Just in case. For example copy it to "/etc/gai.conf.org"
2) Uncomment the following line:
#
# For sites which prefer IPv4 connections change the last line to
#
precedence ::ffff:0:0/96 100
That's it!"This will allow you to still use IPv6 but sets IPv4 as the precedence so that apt-get won’t get stuck."
Thanks Zach!