As mentioned elsewhere, there are some known issues; the team are looking into fixes. In the meantime if you're hit with any issues 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.)