invalid character in base-64 string and invalid postback or callba

.....using VS2005, AJAX 1.0 and Windows Workflow Foundation/.net 3.0

The initial page (with four datagridview controls and 7 UpdatePanels)
Originally I got the error message defined in the AsyncPostBackErrorMessage
attribute on the ScriptManager tag.

After setting EnablepartialRendering="false", I got the "Invalid Postback or
CallBack argument..".

I then set to see the actual error.

I now get "Invalid Character in Base-64 String" and the following stack
trace. I have been to
http://support.microsoft.com/default.aspx?scid=kb;en-us;5553 53 but, as this
is running on my WINXP workstation, the only senario that could fit is
"Form Posts
Viewstate can only be posted back to the same page. Attempting to post an
aspx form to another page will fail with a viewstate invalid exception. This
behavior is by design".

This makes sense to me and leads me to look to the WWF. Any ideas or
suggestions?

stack
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System.Convert.FromBase64String(String s) +0
System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +72
System.Web.UI.LosFormatter.Deserialize(String input) +11

MyCompany.ApplicationBlocks.UIProcess.SessionManagement.Sess ionManagementModule.DeserializeSessionFromPage(String session) +120

MyCompany.ApplicationBlocks.UIProcess.SessionManagement.Sess ionManagementModule.OnAcquireRequestState(Object source, EventArgs args) +231

System.Web.SyncEventExecutionStep.System.Web.HttpApplication .IExecutionStep.Execute() +92
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&
completedSynchronously) +64


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kevin...