Recently I stumbled upon that kind of error when trying to update a submodule:

git submodule update –remote –merge
fatal: unable to access ‘https://github.com/DaveGamble/cJSON.git/’: Received HTTP code 407 from proxy after CONNECT

Turns out that it was the proxy authentification not being sent correctly.
In our case we are using system defined proxy, so we do not need git’s ones

Solved by using:

git config –global http.proxyAuthMethod ‘basic’
git config –global –unset http.proxy
git config –global –unset https.proxy

Some also encountered SSL verification problems due to auto signed certificate use in their network, and had to disable SSL verification. It’ done with the following command, but we do not recommend to use it unless really needed:

git config –global http.sslVerify false