Error message when you view an activity form in Microsoft Dynamics CRM 4.0: "Exception of type 'System.Web.HttpUnhandledException' was thrown"

Error: Exception of type 'System.Web.HttpUnhandledException' was thrown. Error Message: 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. Error Details: Exception of type 'System.Web.HttpUnhandledException' was thrown. Source File: Not available Line Number: Not available Request URL: http://URL
Stack Trace Info: [ArgumentNullException: Key cannot be null. Parameter name: key] at System.Collections.Hashtable.ContainsKey(Object key) at System.Collections.Hashtable.Contains(Object key) at Microsoft.Crm.Metadata.PreloadedMetadataCacheDataProvider.TryGetEntity(String entityName, NameMappingType mapping) at Microsoft.Crm.Metadata.DynamicMetadataCache.GetEntity(String entityName, NameMappingType mapping) at Microsoft.Crm.Security.User.HasPrivilege(IUser user, String entityName, PrivilegeType privilegeType, PrivilegeDepth privilegeDepth) at Microsoft.Crm.Security.User.HasPrivilege(IUser user, String entityName, AccessRights accessRights, PrivilegeDepth privilegeDepth) at Microsoft.Crm.Security.User.HasPrivilege(IUser user, String entityName, AccessRights accessRights) at Microsoft.Crm.Application.Components.Sdk.FormControls.Web.LookupControl.set_LookupTypes(Int32[] value) at Microsoft.Crm.Application.Components.Sdk.FormControls.Web.LookupControl.set_Metadata(AttributeMetadata value) at Microsoft.Crm.Application.ControlActivator.CreateControl(ControlDescriptor controlDescriptor, String clientId) at Microsoft.Crm.Application.Forms.CrudForm.BuildFormModel(FormDescriptor formDescriptor) at Microsoft.Crm.Application.Forms.CustomizableForm.Execute(Entity entity, String formType) at Microsoft.Crm.Application.Forms.CustomizableForm.Execute(Entity entity) at Microsoft.Crm.Web.Activities.BasicActivityPage.ConfigureForm() at Microsoft.Crm.Web.Activities.PhoneCallDetailPage.ConfigureForm() at Microsoft.Crm.Application.Controls.AppUIPage.OnPreRender(EventArgs e) at System.Web.UI.Control.PreRenderRecursiveInternal() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)