VS 2010 Beta 2 problem with System.Web.dll

Question

All of a sudden i'm getting this error in design view in all my webforms (they all use AJAX.NET)

Error Creating Control-ScriptManager1
"Cannot find type System.Web.UI.IScriptManager in module System.Web.dll"
and
Error Creating Control-UpdatePanel1
"Cannot find type System.Web.UI.IUpdatePanel in module System.Web.dll"

Answer 1

Solved.

Replaced System.Web.dll with a partner's and restarted VS