Cannot push to certain repositories


I seem to have messed up my authentification. I use sourcetree and it uses oauth+ssh to connect me with github. I can write and push to repositories from one organization but it does not work when trying to push to other organizations. I am puzzled ad stuck. When I started debugging, I could not even clone new repositories (sourcetree did not display them and complained when I entered the URL manually. I got 403 error). I reregistered via oauth and now I can clone, but I am not able to push:

git push
ERROR: Permission to FOO/bar.git denied to user-name.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

Any help would be really appreciated.