Error message in CiviMail: "Mailing has not been saved, Content maybe out of date, please refresh the page and try again"
I puzzled over this problem after a 4.7.28 upgrade for hours before reading this post. Turning off and on Ckeditor worked for me, which gave me another idea.

Since this happened to me with two sites, I tried clearing all caches on the second site and that did the trick. I suspect changing the ckeditor setting flushes a cache, and flushing the cache is the real fix.

I flushed all the caches I have so I'm not sure which is the one that really does the trick:


drush cc all (only works on drupal)
Rebuild the menus: http://yourdevsite.org/civicrm/menu/rebuild?reset=1 
I deleted all the files in files/css/, files/js/, and /files/civicrm/templates_c/en_US/*