Fixing Craft\HttpException Error Recently moved site to a new server. 

Attempting to edit a post created before migration fails. The edit page reloads with the changes removed. Note: 1) this happens for only SOME, but not ALL of entries in the same channel. 2) the entries that fail and those that succeed are set to use the same templates.

Utilities - Logs shows this error message:

Level     error

Category  exception.Craft\HttpException.404

Message   Craft\HttpException in 
  /home/method/craft/app/controllers/TemplatesController.php:72
That's it. mod_security is on by default and a rule is getting triggered. Turned it off and now the entry can be modified and saved. Thank you Brad!