If you open a web page on one of the websites hosted on our server, leave it for 20 minutes and then submit a form, a Validation of viewstate MAC failed. error occurs.
What possible reasons could there be for this?
It's taken us a while to find the answer to this as I had been informed that another IIS7 server I was comparing it to had been setup in the same way, by the same person.
It turns out the server with the websites which were receiving this error had been setup using Plesk, whereas the other server had not been.
It seems Plesk sets the Idle-Timeout to 5 minutes on the application pools, which is what was causing this error.
To change this do the following: