Suddenly all my permalinks stopped working. I can access the posts from the Dashboard editor but ALL of the links produce a 404 (404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.)
This only happens on this blog, not on my other (http://sherwoodmonastery.sageline.com/). Both blogs use the same provider (though different servers) and have WPMU installed.
!!! While not comfortable going into the htaccess file I followed the suggestion of http://www.wpbeginner.com/ and added some code to the WordPress section and it fixed the problem. Sorry for the false alarm.