R – WebResource.axd – Invalid ViewState

I seem to recall that this error can occur if you click a button/link etc before the page has fully loaded.

If this is the case, the error is caused by an ASP.net 2.0 feature called Event Validation. This is a security feature that ensures that postback actions only come from events allowed and created by the server to help prevent spoofed postbacks. This feature is implemented by having controls register valid events when they render (as in, during their actual Render() methods). The end result is that at the bottom of your rendered form tag, you'll see something like this:

<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION"  value="AEBnx7v.........tS" />

When a postback occurs, ASP.net uses the values stored in this hidden field to ensure that the button you clicked invokes a valid event. If it's not valid, you get the exception that you've been seeing.

The problem you're seeing happens specifically when you postback before the EventValidation field has been rendered. If EventValidation is enabled (which it is, by default), but ASP.net doesn't see the hidden field when you postback, you also get the exception. If you submit a form before it has been entirely rendered, then chances are the EventValidation field has not yet been rendered, and thus ASP.net cannot validate your click.

One work around is of course to just disable event validation, but you have to be aware of the security implications. Alternatively, just never post back before the form has finished rendering. Of course, that's hard to tell your users, but perhaps you could disable the UI until the form has rendered?

from http://forums.asp.net/p/955145/1173230.aspx

The error is because your appdomain was recycled/restarted. When that happens the application and the machine key is set to auto, it changes. That affects the decryption of the info in the url of the resources urls (.axd). Setting up a fixed machine key will prevent it from ever happening again.

Please check this for more info on a similar case (the explanation is with an issue with viewstate validation, but the cause is the same one): http://www.developmentnow.com/blog/InvalidViewstate+Or+Unable+To+Validate+Data+Error.aspx

I also had been wondering about it for quite a while. After I saw this question it got me on it again: Is this an attempt to break my ASP.Net site's security? ... which I just answered with very much the same. I had the feeling it was around some restart of something, because when we published something that recycled the application the error showed in the log, but I didn't have any other source stating its relation (today I found that case on invalidviewstate because of the machinekey change :))

Ps. above finally explains it on single server :)