had the same problem with celtx 2 on mac, found the solution online:
1. Quit Celtx
2. Then Navigate to: /Home/Library/Application Support/
(Home being your mac username)
3. Then delete the Celtx folder that resides there. (or my advise to rename it)
4. Re-Start Celtx
all done
you have to browse for your projects though… recent projects will be empty as its a fresh install