ViewState (v1.1) gets corrupted after 978th characters

[Jerry]

Hi, I've been getting dozens of the same exceptions every week from our web
server (running asp.net v1.1).

I looked at the viewstate in the exception, all of them shared the same
problem: having some extra characters (0x21 0x0d 0x0a 0x20, which is "!\r\n
") inserted after the 978th character.

There's no server farm or load balancing and the problem came from all major
browsers (e.g. IE, Firefox) so I'm suspecting some kind of browser plug-in
or maybe the firewall, anti-virus software did this to the hidden form
field.

Does anyone know the cause of this problem?, or even a cure?

TIA,
Jerry

Here's the exception stack:

HttpException: "Invalid_Viewstate ...
....

HttpException: "Authentication of viewstate failed. 1) If this is a cluster,
edit <machineKey> configuration so all servers use the same validationKey
and validation algorithm. AutoGenerate cannot be used in a cluster. 2)
Viewstate can only be posted back to the same page. 3) The viewstate for
this page might be corrupted."
at System.Web.UI.LosFormatter.Deserialize(String input)
at System.Web.UI.Page.LoadPageStateFromPersistenceMedium()

HttpException: "Unable to validate data."
at System.Web.Configuration.MachineKey.GetDecodedData(Byte[] buf, Byte[]
modifier, Int32 start, Int32 length, Int32& dataLength)
at System.Web.UI.LosFormatter.Deserialize(String input)

 

[Winista]

Some proxy servers or web accelrators tend to chop up the hidden fields if
they are larger than certain size. Check your web log for these errors and
see if you can see some kind of proxy server installed for these users.