Hi. Some days ago I updated our Umbraco 3 installation to the newest (4.0.3.1). Yesterday I discovered that I - in some ways - can't create new nodes in the Media tree. If I select the create button in the top of the media panel all works out well, but if I right click the node where I would like to create the childnode, and selects "Create" I get the following error message in the popup panel:

Server Error in '/' Application.
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ERROR CREATING CONTROL FOR NODETYPE: Media 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
Exception Details: System.ArgumentException: ERROR CREATING CONTROL FOR NODETYPE: Media
Source Error: 
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  
Stack Trace: 
[ArgumentException: ERROR CREATING CONTROL FOR NODETYPE: Media]
   umbraco.cms.presentation.Create.Page_Load(Object sender, EventArgs e) +415
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
   System.Web.UI.Control.OnLoad(EventArgs e) +99
   umbraco.BasePages.BasePage.OnLoad(EventArgs e) +15
   System.Web.UI.Control.LoadRecursive() +50
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627
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Version Information: Microsoft .NET Framework Version:2.0.50727.3603; ASP.NET Version:2.0.50727.3082 

 Anyone got any help to solve the problem?! Thanks!