After successfully cloning my repo from heroku and added another remote
1/ git clone git@heroku.com:[APP].git
2/ git remote add bitbucket ssh://git@bitbucket
TLDR: ssh-add ~/.ssh/yourkey
I've just worked through this problem.
And none of the other answers helped.
I did have a ./ssh/config with all the right stuff, also an earlier repository working fine (same bitbucket account, same key). Then I generated a deploy_key, and after that created a new repository.
After that could not clone the new repo.
I wish I knew how/why ssh agent was messing this up, but adding the key solved it. I mean adding the key in my local Ubuntu, not in bitbucket admin. The command is just
~/.ssh$ ssh-add myregualrkey
Hope this helps someone.