Normally in developer mode Webpack runs using HTTP. There is usually a web server serving content through HTTP and webpack using http/websockets on a separate port.
this for TEST environment only:
you need to configure your webpack-dev-server as follows:
webpack-dev-server --https --cert ./cert.pem --key ./key.pem
The easiest work around is to generate a key with no passphrase (I don't know the security consequences of this! but this is for test only) .
To take the passphrase out of your key use this command:
$ openssl rsa -in key.pem -out newKey.pem
and use the new key in the previews configuration line