Getting a long PageRequestManagerParserErrorException error with some HTML in it when using ASP.NET Ajax with something like a Gridview? Does the error only happen once in a while, usually the first time you do something specific after a long delay?
Know you're not actually using Response.Write and only using MS controls?
Well, maybe asp.net is trying to do a postback for you to create a cookie when you are using the session variable.
Solution: A dirty solution was calling Session["IWantASessionCookie"] = true; on a page load, after this the problem went away as ASP.NET didn't need to create the local cookie when doing things in the UpdatePanel as it had already done it.