Whenever I try to edit a commit on the rebase window, I get a "rebase --abort" right after I leave the screen.
I'm following the instructions to edit a commit found here (checking the "ammend commit" checkbox): http://blogs.atlassian.com/2014/06/interactive-rebase-sourcetree/
I ended up doing this by command line, and everything worked as expected, so I believe this a bug in SourceTree. I also tried with many repositories and got the same thing.
I also reported this in Atlassian Answers: https://answers.atlassian.com/questions/321956/interactively-rebase-always-aborting