View state issues with the error “Validation of viewstate MAC failed”. I ran across this error while building a report in ASP.net c# web forms. The error first started when a viewer refreshed the screen after a long period of no use. Evidently this is a common issue with dotNet apps located on a server instance that also houses several other applications within the same instance (same issues as those experienced with a Web Farm). One fix I found to get around this issue is to switch from auto-generated keys to defining static complex keys.
The following is generated from a completely random key generator that I have developed. Refresh the screen to generate a new set of keys.
validationKey is SHA1 and 128 bits
decryptionKey is AES and 64 bits
Add this to your web.config
Adapted from this MSDN article on MachineKey Configuration.