I found the line
quagga quaggavty 775 /etc/quagga
in the file
After deleting that line, I was able to install
Apparently, the oficial way to do it is (this also happened later on another server, and this time I used this form):
sudo dpkg-statoverride --remove /etc/quagga
And to list all overrides:
From man dpkg-statoverride
Remove an override for path, the status of path is left
unchanged by this command.
List all overrides. If a glob pattern is specified restrict
the output to overrides which match the glob. If there are no
overrides or none match the glob dpkg-statoverride will exit
with an exitcode of 1.