Sourcetree commit button does nothing

I'm running version 3.0.8 for Windows, and when I press Commit button, the "Checking submodules" dialog shows for half a second, and then nothing. No error message, and nothing has been commited. I can commit using the command line, however.

I previously "solved" this problem by uninstalling, deleting the userdata, and reinstalling (a simple reinstall was not sufficient).

But this is getting tedious. How can I get to the bottom of this?

Update: The log file says:

ERROR [2018-11-19 12:17:28,440] [49] [SourceTree.ViewModel.RepoDialogContainer] [Log] - The calling thread must be STA, because many UI components require this.

System.InvalidOperationException: The calling thread must be STA, because many UI components require this.

at System.Windows.Input.InputManager..ctor() at System.Windows.Input.InputManager.GetCurrentInputManagerImpl() at System.Windows.Input.KeyboardNavigation..ctor() at System.Windows.FrameworkElement.FrameworkServices..ctor() at System.Windows.FrameworkElement.EnsureFrameworkServices() at System.Windows.FrameworkElement..ctor() at System.Windows.Controls.Control..ctor() at SourceTree.View.Controls.DialogFileListControl..ctor(IEnumerable`1 items) at SourceTree.ViewModel.CommitAndFileStatusViewModel.CreateDisabledStagingViewCommitProcess(CommitOptions commitOptions, String useAuthor) at SourceTree.ViewModel.CommitAndFileStatusViewModel.FinaliseCommit() at SourceTree.ViewModel.CommitAndFileStatusViewModel.SubmoduleDialogCompleted(AbstractDialogViewModel repoDlg, Int32 returnCode) at SourceTree.ViewModel.AbstractDialogViewModel.Finish(Int32 returnCode) at SourceTree.ViewModel.SubmoduleCheckDialogViewModel.<RefreshSubmodules>d_46.MoveNext()