I am behind a firewall and RubyMine is not able to push to GitHub.
RubyMine has its proxy configuration set and it can perform a few network operations, which show that the proxy configuration is indeed working - but Git cannot push to GitHub.
Git under RubyMine does not use the RubyMine proxy configuration.
The proxy configuration for Git should be done directly on git through the command line:
git config --global http.proxy server:port git config --global https.proxy server:port
I think the same should apply to other JetBrains tools (like IntelliJ which I also use).