i keep seeing the above error in my log file anytime i view it ... please i would like someone to help me with the cause and how to solve it

Search the forums for adding the machineKey and decryptionKey settings to your web.config -- your application pool is shutting down in IIS and in ASP.NET, if no machineKey and decryptionKey are present in the web.config, the server creates new 'hashes' that encode the viewstate (the encrypted form data) used to run the application. When the encryption changes, the client's connection is basically reset and the old form data is no longer valid.

Agreed - and for what it's worth, it doesn't cure it completely. Spammers will many times try to send http-POST requests to sites hoping to get around security or permissions and just 'post' to a site -- this will almost always show errors in your logs. DotNetNuke has the same problem, mostly due to spammers.

The machineKeys do make a huge help, in regards to your site 'timing out it's application Pool while a user is say posting or reading though. It tends to help the 'user experience' quite well, just doesn't resolve the issue when it comes to spammers --