This is written in the log:

2013-09-16T17:48:29: Fatal error:
System.NullReferenceException: Object reference not set to an instance of an object.

2013-09-16T17:48:30: Stack trace:
at SourceTree.Model.Repository.AutodetectRemoteProjectLinks()
at SourceTree.AppRoot.UpgradeRemoteLinks()
at SourceTree.AppRoot.DoWindowStartupTasks()
at SourceTree.ViewModel.MainWindowViewModel..ctor()
at SourceTree.MainWindow..ctor()
at SourceTree.AppRoot.OnStartup(StartupEventArgs e)
at SourceTree.App.OnStartup(StartupEventArgs e)
at System.Windows.Application.<.ctor>b__1(Object unused)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)