I just started getting this error today: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. and I haven't changed anything to my web.config file or IIS for over 2 weeks!  I have read a lot of speculation about what it could be but wonder if anyone had an actual fix for this.  What seems to happen is I can get into the portal add a module (text/html, announcements, etc) and I can get in to add or edit text or add an announcement and when I click save or update I get this error. 

The site appears to be working fine otherwise.  It is happening in both my development and production environments (on seperate servers in the same network)  Another funny twist is it is only happening on one portal in both dev and prod environments.  I have several other portals and they are working fine.

Currently running Windows Server 2003, IIS6 with DNN 4.8.2 and SQL 2005. 

To add, I had found information from Microsoft stating it was a .NET 2.0 thing and checked and we did not have SP1 installed for .NET 2.0.  I installed in on our server and still have the same problem.  Again, only on the one portal.