I found the problem.
Apparently, an earlier installation had modified my /etc/hosts file . There was a line specifying an incorrect IP address for deb.nodesource.com. Removal of this line and a reboot solved the problem. Thanks for the help.
That suggests that wget and apt do not pay attention to /etc/hosts, which seems very odd.
Don't forget to run
sudo apt update && sudo apt full-upgrade
to make sure all your system packages are up to date. Presumably this has not been done for some time.
