Update
Ran into this again today - when my work laptop was taken home - appears that a Window domain might be the underlying issue. Found the soluti
I ran into this same issue today. My private key wasn't owned by the account I was logged in with.
For anyone else make sure that your account is the owner of the key. In my case it wasn't as it was copied from another account.
Running ssh -i /path/to/key -vT git@github.com as noted by @lum showed me that I didn't have permission to access my private key. Changing owner on the key fixed my issue.
debug1: could not open key file '/home/matt/.ssh/id_rsa': Permission denied
Hope this helps future people.