If you have problems with the connection between SourceTree and your git repo over SSH, this post should be useful for you…

A common error is:
“FATAL ERROR: Server unexpectedly closed network connection
fatal: Could not read from remote repository.”

In order to connect to the git repo over SSH, I advise to install the complete PuTTY package from the official WebSite.
After this, you can copy the latest executables from the PuTTY folder (Depending on the version, usually: C:\Program Files\PuTTY) to the SourceTree folder (Depending on the version, usually: C:\Users\\AppData\Local\SourceTree\app-2.0.20.1\tools\putty)

• plink.exe
• (pageant.exe)
• (puttygen.exe)
• (putty.exe)

Now you can configure normally the connection in SourceTree and with this latest version of plink it will work.