After upgrading to .NET4, I get YSOD, when creating scriptManager

Recently, I upgraded an existing application to .NET4. It compiles and runs, but after login screen, when I get to the main page, I get a YSOD, stating:

Could not load file or assembly     'Microsoft.Practices.EnterpriseLibrary.Configuration.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=开发者_运维百科31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified. 

The highlighted line is this

 <asp:ScriptManager ID="ScriptManager1" runat="server">

If I reload the page, it works like a charm, until the AppPool is restarted - then the story repeats.

Do any of you have any idea, why this could be happening?