Its not actually an error. Its a message telling you that you cannot view the error.
You have to do the following to resolve this problem.
First setting in the web.config will display you the actual errror. Also check if you have the correct .NET version installed on the server and your \test folder is configured to the right .net version.
If still getting the same error the do the following:
In the IIS, right click on your website and in the properties dialog, click on the ASP.NET tab and set the correct version. For example see the following snap.
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