Upgrading to ASP.NET 2.0 causes issues with LinkButton

I am running into an issue after upgrading my ASP.NET 1.1 project with
regards to link buttons. Clicking on any link button produces the error:
Unable to validate data.
Stack Trace: at
System.Web.Configuration.MachineKeySection.GetDeco dedData(Byte[] buf, Byte[]
modifier, Int32 start, Int32 length, Int32& dataLength)
at System.Web.UI.ObjectStateFormatter.Deserialize(Str ing inputString)

I have noticed that the javascript for the link has changed from

if (typeof(Page_ClientValidate !)= 'function' ||Page_ClientValidate ())
_doPostBack("link1","") -- in .NET 1.1

to

WebForm_DoPostBackWithOptions(new
WebForm_PostBackOptions("link1","",true,"","",fals e,true)) -- in .NET 2.0

Any help on how to solve this would be greatly appreciated.