ViewState times out after 15 minutes... "Invalid Viewstate"
<p>I'm getting a strange error for some users on a website I created.</p>

<p>This exception is thrown occasionally:</p>

<pre><code>System.Web.HttpException (0x80004005): The state information is invalid for this page and     
might be corrupted. ---&gt; System.Web.UI.ViewStateException: Invalid viewstate. 

[...]
---&gt; System.ArgumentException: The serialized data is invalid. ---&gt; 
System.Runtime.Serialization.SerializationException: Unable to find assembly 
EntityFrameworkDynamicProxies-HROpenEnrollment.Model, Version=1.0.0.0, Culture=neutral, 
PublicKeyToken=null'. at 
System.Runtime.Serialization.Formatters.Binary.BinaryAssemblyInfo.GetAssembly() at 
[...]
</code></pre>

<p>Upon talking to the user that encountered this error, he said he simply opened the page, waited around 15 minutes, and then started using it when he received this error. Once refreshing, everything works fine. A few other users have encountered this error but they did not even notice it.</p>

<p>The page stores a form object in the viewstate during the duration of the page, and when the user submits the page, the form is pulled from the viewstate and sent to the server. What would cause that viewstate to become invalid?</p>

<p>I looked on google and found an article about this from microsoft, but it doesn't seem to be related to my problem.</p>
