Today I got this error when using github:
$ git pull origin master
I googled it and found the solution here.
For most users, simply running ssh-add to load your keys into the SSH agent will fix this issue.
If your key does not have the default filename, you’ll have to pass the path to “ssh-add”
$ ssh-add ~/.ssh/my_other_key
The interesting part of this error is: it only happens in Debian/Ubuntu systems.