I have a TwentyEleven site with all plugins disabled and I am still getting a 'page not found' when I try to add a new custom menu.
I updated to 3.4.2 form a slightly older version and this did not fix the problem.
I tried a different theme and this also did not fix the problem.
I tried going back to the default htaccess file from my more secure version and this did nothing either.
the site is http://www.1.geomagnetballs.com
on a HG shared server.
I am not a programmer so please keep suggestions suitable for dummies or give clear instructions :wink: - Of course I can give you access if you would care to look inside ...