Hey there,
I'm using the latest SourceTree (Standalone, no plugin) and Visual Studio 2015. I'm not able to stage files or commit while Visual Studio is running. The error says: "The OpenSSH agent is running but the server still rejected your connection. You may need to add the correct key to your agent... ". Using the terminal (git add *) works fine, so this seems to be a problem of SourceTree. After closing Visual Studio everything works, too.
Thanks for looking into this. By the way, I'm able to reproduce this behaviour on three machines.
Kai