A newer version of Default App is already running.
The version of Default App that loaded at startup is newer than the one youΓÇÖre trying to use now.
Default App canΓÇÖt work on this system.
Some software is conflicting with the operation of Default App. To avoid further problems, Default App will disable itself until the conflicting software is removed.
Default App has a problem.
Some system-level data structures used by Default App are corrupt for some reason. Try rebooting the computer and see if the problem goes away.
Mac OS Easy Open is not installed.
Default App works better if Mac OS Easy Open is installed (even if itΓÇÖs turned off). It will work anyway now, but document kinds may not be displayed correctly.