Can't clone repo - Askpass could not be started

When I try to clone a git repository from Bitbucket, Sourcetree opens a dialog box:

"""
Askpass.exe - This application could not be started.

This application requires one of the following versions of the .NET Framework:
v.4.0.30319

Do you want to install this .NET Framework version now?
"""

I click yes, then a web page opens and start the download of .NET Framework v4.5.1. When the download runs, it tells me that that version of .NET Framework is already installed.

I also searched version 4.0.30319 from the MS site and downloaded it, but when installing it also says that it is already installed.

Doesn't SourceTree work in windows 8.1 ?

Thanks