Don't know but corrupt preferences can create a vast range of different symptoms, so whenever FCE stops working properly in any way, trashing the preferences should be the first thing you do using this free app.
Shut down FCE, open PreferenceManager and in the window that appears:-
1. Ensure that only FCE is selected.
2. Click Trash
The job is done instantly and you can re-open FCE.
There is absolutely no danger in trashing preferences and you can do it as often as you like.
The preferences are kept separately from FCE and if there aren't any when FCE opens it automatically creates new ones . . . instantly.