Update: Pushes to contributed modules and sandboxes are once again possible, thanks to the work of Sam Boyer and Damien Tournoud. Go forth and contribute!
We are experiencing technical problems with repositories hosted on the Drupal.org git server.
The Drupal.org infrastructure team has been informed and is working to resolve the issue.
Anonymous cloning and pulling from the repositories is still possible. Short service interruptions may occur while the team is working on the problem.
Our apologies for disrupting your work.