One of the security fixes for 4.8.2 was http://www.dotnetnuke.com/News/SecurityBulletins/SecurityBulletinno12/tabid/1148/Default.aspx which updated the validationkey if it was a known value. The message you're getting usually indicates either a page using an "old" validationkey , in which case close the browser and start fresh. If you're using multiple webservers then please make sure that the key is the same in each web.config.

Cathal

Never mind...  After re-reading the notice above it over and over, I finally got it.  For some reason, it wasn't sinking in immediately.  For others like me who may come across this post...  Leave the "AutoGenerate,Isolate" unless you are using a Web Farm.  When running in a web farm environment, you need a validation key, and it should be the same across all of your web.configs (under the same site).