+[NSUndoManager(NSInternal) _endTopLevelGroupings] is only safe to invoke on the main thread.
The problem here is that Qt 4.6 for 64 Bit is (rightly so) built using the Cocoa API, instead of the C-only Carbon API. Now it seems that Qt uses the NSUndoManager, which in turn has an undocumented (?) function _endTopLevelGroupings. And as the above mentioned message tells you, it is only safe to call that function from the main thread. Since the qt4-gui plugin of Licq runs in a separate thread, this assertion fails, and the whole program crashes. So this basically means you cannot start up your GUI from a secondary thread with Cocoa based Qt. This, I think, constitutes a bug in Qt.
Update: I submitted a bug tracker item, and erijo is producing a minimal example that will hopefully show the bug in action. Now let's wait and see.