For some reason Bitbucket thinks that I am the account "lobu" and not "lobu_tpr". I added a newly created public SSH key to my personal account. This account has write access to "lobuadmin". I cloned a repository to my machine using the SSH url and commited some test change and pushed it to the Bitbucket repository. The website tells that I am "lobu" not "Tobias Preuss" aka. "lobu_tpr".
To clarify: I had another! SSH key added to the "lobu" account before. But I removed this! Further "lobu" had write access to "lobuadmin" but this has been removed too!