Viewstate becomes corrupt intermittently

I have had a user of my VB.net web app complain he keeps getting an intermittent error message, causing him to crash out of the system.

Stack Trace:

[FormatException: Invalid character in a Base-64 string.]
System.Convert.FromBase64String(Strings) +0
System.web. UI. LosFormatter.Deserialize(String input) +25
System.web. UI. Page. LoadPageStateFromPersistenceMedium() +101
[HttpException (0x80004005) : Invalid_Viewstate
Client IP: 127.0.0. 1开发者_运维百科
Port: 3491
User-Agent: Mozilla/4.0 (compatible: MSIE 7.0; windows NT 5.2; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.648; .NET
ViewState: dDwxMZUZMDI3MTC203Q802w8aTwxPjS+02w8dDw7bDxpPDU+02  kBNj47aTw4PjtpPDEDPjtpPDE4PjtpPDIwPjtpPDIXPjtpPDM  OPjtpPDM1PjtpPDM2PjtpI
Http-Referer: http ://localhost/<appname>/webForms/frmSystem.aspx?User=2&systemNo=22011U&Zoom=1&Strips=6
Path: /<appname>/webForms/frmSystem.aspx.]
System.web. UI. Page. LoadPageStateFromPersistenceMedium() +442
System.web. UI. Page. LoadPageViewState() +18
System.web. UI. Page. ProcessRequestMain() +447

It appears from the error message that the viewstate keeps becoming corrupt (see screen-dump below), but I don't understand why. He claims it happens every 2-3 minutes, but I've used the system a lot and have never experienced it.

I should mention too that he is using the web-app in a very "non-standard" way. To achieve optimum speed he's actually using remote desktop, to connect to our server directly, and running it as localhost. ("Madness!"- yes, I know. I didn't make the decision.) That may not be relevant of course.

Some advice on what is likely to be causing the viewstate to become corrupt would be extremely helpful.

Screen-dump of error

He is double clicking. Same thing drove me nuts for a long time because I know how many times to click on things and never saw the error myself.