validation of viewstate MAC failed in ASP.NET Web Form

validation of viewstate MAC failed

There are few reason for this error. Below is one of them.

Check the idle time of application pool in IIS and try to increase.

1. Open IIS
2. Click on application pools node
3. Locate your web application's application pool
4. Right-Click and select Advanace Settings
5. Set the Idle Time-out(minutes) property to 0 or increase 

Ref:
stackoverflow