What can 'be the cause?

Server Error in '/' Application.

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Compilation Error

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0234: The type or namespace name 'ForumPages' does not exist in the namespace 'YAF.Classes.Utils' (are you missing an assembly reference?)

Source Error:

Line 36: private string _origHeaderClientID;

Line 37: private string _origFooterClientID;

Line 38: private YAF.Classes.Utils.ForumPages _page;

Line 39: public event EventHandler PageTitleSet;

Line 40:

Source File: d:\inetpub\vhosts\synologyitalia.com\httpdocs\App_Code\YAF\Forum.cs Line: 38

Compiler Warning Messages:

Warning: CS0436: The type 'YAF.RegisterForum.RegisterForumCompletedEventHandler' in 'c:\windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\0de4b011\437c7615\App_Code.cgxu2gdp.5.cs' conflicts with the imported type 'YAF.RegisterForum.RegisterForumCompletedEventHandler' in 'c:\windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\0de4b011\437c7615\assembly\dl3\e397a7aa\5e705062_108bcb01\YAF.DLL'. Using the type defined in 'c:\windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\0de4b011\437c7615\App_Code.cgxu2gdp.5.cs'.

Source Error:

d:\inetpub\vhosts\synologyitalia.com\httpdocs\App_Code\YAF\Web References\RegisterForum\Reference.cs

Line 75:

Line 76: ///

Line 77: public event RegisterForumCompletedEventHandler RegisterForumCompleted;

Line 78:

Line 79: ///

.........

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Version Information: Microsoft .NET Framework Version:2.0.50727.4952; ASP.NET Version:2.0.50727.4955