npm behind a proxy fails with status 403

OK, so within minutes after posting the question, I found the answer myself here: https://github.com/npm/npm/issues/2119#issuecomment-5321857

The issue seems to be that npm is not that great with HTTPS over a proxy. Changing the registry URL from HTTPS to HTTP fixed it for me:

npm config set registry http://registry.npmjs.org/

I still have to provide the proxy config (through Authoxy in my case), but everything works fine now.

Seems to be a common issue, but not well documented. I hope this answer here will make it easier for people to find if they run into this issue.

If you need to provide a username and password to authenticate at your proxy, this is the syntax to use:

npm config set proxy http://usr:pwd@host:port
npm config set https-proxy http://usr:pwd@host:port

npm config set proxy http://proxy.company.com:8080
npm config set https-proxy http://proxy.company.com:8080

credit goes to http://jjasonclark.com/how-to-setup-node-behind-web-proxy.