After upgrade to new version of SSH you may experience the following problem with return code 255:
This happened to me after upgrade from Linux Mint 17 to 18.
How to diagnose the problem?
Run following command:
You will see several lines of output and one of them is typically the reason. In my case it was:
DSA keys were disabled in SSH 7. Solution is to generate new key with different type, for example RSA.
Replace your key at GitHub by newly generated one. You can re-run “ssh -v” command to check whether the communication was established correctly:
That’s a good sign. The connection was established and now you can try to clone a repo.