Request for the permission of type XXX failed

During the creation of a new Web Application I had a bit of a problem. I forgot to change the trust level. I got exceptions from every webpart that looked like this example:

Exception in UserControl:
System.Security.SecurityException: Request for the permission of type 'Microsoft.SharePoint.Security.SharePointPermission, Microsoft.SharePoint.Security, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' failed. at Microsoft.SharePoint.ApplicationRuntime.SPVirtualPathProvider.GetCacheKey(String virtualPath) at Microsoft.SharePoint.Publishing.Internal.CmsVirtualPathProvider.GetCacheKey(String virtualPath) at System.Web.Compilation.BuildManager.GetCacheKeyFromVirtualPath(VirtualPath virtualPath, Boolean& keyFromVPP) at System.Web.Compilation.BuildManager.GetVPathBuildResultFromCacheInternal(VirtualPath virtualPath) at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath
virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) at System.Web.UI.TemplateControl.LoadControl(VirtualPath virtualPath) at xxx.customclass.method() The action that failed was: Demand The type of the first permission that failed was: Microsoft.SharePoint.Security.SharePointPermission The Zone of the assembly that failed was: MyComputer

If you encounter this exception then go to the web.config in the folder of your web application:

C:\Inetpub\wwwroot\wss\VirtualDirectories\<webappdir>

Then change the <trust level=" - tag as shown below.

Change:
<trust level="WSS_Custom" originurl="">

to:
<trust level="Full" originurl="">