Validation Of Viewstate MAC failed

If it is a webfarm with dynamic load balancing, you may see this error. Basically what is happening is, a server is serving the request for a page and the page is posted back to a different server. The two servers use different keys to encrypt the viewstate information. The server that receives the postback request tries to decrypt the viewstate with its key which is different from the key it as encrypted. To fix this issue, you have to use the same key across all servers in your webfarm.
You can set this either in machine.config or your web.config. But it has to be the same across all servers.

<machinekey validationkey="<encryptionkey>" decryptionkey="<decryptionkey>" validation="SHA1" decryption="Auto" />

You can use this C# function to generate the keys:

public static string CreateMachineKey(int length)
		{
			// Create a byte array.
			byte[] random = new byte[length/2];
			// Create a cryptographically strong random number generator.
			System.Security.Cryptography.RNGCryptoServiceProvider rng = new System.Security.Cryptography.RNGCryptoServiceProvider();
			
			// Fill the byte array with random bytes.
			rng.GetBytes(random);
			
			// Create a StringBuilder to hold the result once it is
			// converted to hexadecimal format.
			System.Text.StringBuilder machineKey = new System.Text.StringBuilder(length);
			// Loop through the random byte array and append each value
			// to the StringBuilder.
			for (int i = 0; i < random.Length; i++)
			{
				machineKey.Append(String.Format("{0:X2}", random[i]));
			}
			return machineKey.ToString();
		}

The encryption key length can be between 40 and 128 and the decryption key lengh can be either 16 or 48. The recommended lengths are 128 for the encryption key and 48 for the decryption key.