The web page at http://logback.qos.ch/repos.html says the main git repo can be cloned by doing:

git clone http://git.qos.ch/logback/

This fails with:

warning: remote HEAD refers to nonexistent ref, unable to checkout.

And indeed http://git.qos.ch/logback/refs/heads/master points to 521dfe290dfa34fadb247ee63607a0cddaaf181f
which doesn't seem to exist in http://git.qos.ch/logback/objects/

cloning from github works fine