Resolving view state message authentication code (MAC) errors

This problem happens when your asp.net application runs on web farms
Adding enableViewStateMac="false" to webform may solve the problem but it's not recommended.

BY IIS 7

<?xml version="1.0" encoding="UTF-8"?> 
<configuration> 
    <system.web> 
        <machineKey decryptionKey="xx,IsolateApps" validationKey="xxx,IsolateApps" /> 
    </system.web> 
</configuration>
To generate using Power shell use this instruction http://support.microsoft.com/kb/2915218

Copy and paster powershell script into file, while running you may get security error.
To remove security try

x86
Open C:\Windows\system32\cmd.exe
Run the command powershell Set-ExecutionPolicy RemoteSigned

x64
Open C:\Windows\SysWOW64\cmd.exe
Run the command powershell Set-ExecutionPolicy RemoteSigned

and to set back to default 
powershell Set-ExecutionPolicy Restricted

Power shell Execution Policies