Prevent Validation of ViewState MAC failed error in ASP.NET Web Forms

• 1 Ensure you have machineKey in your web.config file
  
• N That is the encryption codes and methods IIS uses to encrypt and decrypt the viewstate. if you dont have them, then when a new session starts, it generates them and stores them in RAM which the other servers in the farm cant obviously read and therefore dont know the codes to decrypt the viewstate.
  

References

• http://www.codeproject.com/Articles/221889/How-to-Generate-Machine-Key-in-IIS7