You are right! I removed every instance of NPM and Node(js), literely every file. Thereafter reinstalled with then did a dist-upgrade and voila: latest NPM and latest Node(Js) AND after re-installing with the new NPM: a working Easy-Server!
Thanks!

I updated as suggested, unfortunately I now get a:
module.js:549
throw err;
^
Error: Cannot find module '@xmpp/connection'

When running easy-server.
I tried cleaning, reinstalling, reinstalling NodeJS (8.11.2), but until now no success.

Any help appreciated.

NodeJS 8.11.2
NPM 2.15.11
easy ...