Try making a new export destination. (File > Share > Add Destination, and then drag “Export File” to the list on the left). Rename it something cool, setup the parameters you’d like, and try sharing again using that setting.
If that doesn’t work, try trashing the “com.apple.FinalCut.UserDestinations.plist” in the User > Library > Preferences folder. To get there go to Finder, Hold option, select the “Go” menu while still holding option, and then choose Library and Navigate the preferences folder.