Was running version 2.3.2 (75) on Mac OS 10.12.1 just fine. I have repositories on github and bitbucket. After upgrading to SourceTree 2.4, I immediately get this error:
There is an issue with your SSH configuration
The SSH configuration for your GitHub account ((null)) is incomplete. Please edit this account to generate a new SSH key.
When I click the Show Accounts button, no accounts are visible.
If I pull the older version 2.3.2 out of the trash and run it, things are working again.