I have a page with UpdatePanel and ASP:Timer, that refreshs the UpdatePanel every minute.
It was noticed that after new release after the first refresh any postback causes
"Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster."
I've searched Google and found that this error is quite common. Below there are just a few suggestions:
I do not consider viewStateEncryptionMode ="Never" enableViewStateMac="false", because it is asecurity risk.
blog Validation of viewstate MAC failed error:
It appears when using built-in databound controls such as GridView, DetailsView or FormView which utilize "DataKeyNames".
3. It appears if you have a large page which loads slowly for any reason.
* sample code to override the Render Event of the page to place the hidden fields for Encrypted Viewstate and Event validation on the top of the form.
* mark the form as disabled and then enable it in script once the load is complete.
to your Button which triggers the Postback Event:
<meta http-equiv="Page-Enter" content="RevealTrans(Duration=0,Transition=0)" />
Essentially, including this tag in the header of your page disables the use of the page until it has completely downloaded to the browser.
Links related to debugging/viewing ViewState