I’ve just had a very similar problem, where I couldn’t get npm to work behind our proxy server.
My username is of the form “domain\username” – including the slash in the proxy configuration resulted in a forward slash appearing. So entering this:
npm config set proxy "http://domain\username:password@servername:port/"
then running this npm config get proxy returns this:
http://domain/username:password@servername:port/
Therefore to fix the problem I instead URL encoded the backslash, so entered this:
npm config set proxy "http://domain%5Cusername:password@servername:port/"
and with this the proxy access was fixed.