This week I ran into an interesting problem. When adding a workflow to a content type we saw the following error:
Unable to validate data. at System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength) at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString)
I did the normal thing at searched on Google, only to find this KB (http://support.microsoft.com/kb/928028) which describes the error message, it gives the solution as ensuring that SharePoint is installed with the same path on each of the server. This seemed odd, since all our servers have a standard build.
The error message indicated that the view state has been modified between postbacks, so it was back to the drawing board looking for a solution that involved some change that could possibly have some effect on the view state. After talking with the designer it turned out that he had made some changes to the application.master page. We replaced the altered file with a backed up version of the application.master and sure enough the workflow could be added without any issues.
After a bit more experimentation it turned out that the problem was with this:
<SharePoint:DelegateControl ID="MyDelegateControl" runat="server" controlId="SmallSearchInputBox" />
It seems that the small search box alters the view state in some way to cause it not to validate on postback.
Hopefully this will save someone else the hassle of this error.