Yeah, it's a bit of a mess, the team are looking into fixes. In the meantime it appears a downgrade is a reasonable workaround. Links to known-good versions:
http://downloads.atlassian.com/software/sourcetree/SourceTre...
https://downloads.atlassian.com/software/sourcetree/windows/...
(Disclaimer; I'm Atlassian employee, although not on Sourcetree so I don't have any special insight.)