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After you upgrade from Microsoft Dynamics CRM 3.0 to Microsoft Dynamics CRM 4.0, lookup attributes are inactive on the activity form. For example, the To lookup attribute and the From lookup attribute on the E-mail form are inactive.
The lookup attribute was customized in Microsoft Dynamics CRM 3.0. The customization updated the field XML to contain additional field parameters. The additional field parameters are incorrectly duplicated during the upgrade to Microsoft Dynamics CRM 4.0. These duplicate values cause the metadata lookup to display the lookup attribute as inactive.
This problem is fixed in the latest cumulative update rollup for Microsoft Dynamics CRM 4.0. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
949256 Microsoft Dynamics CRM 4.0 updates and hotfixes
To verify that you are experiencing the problem in the "Symptoms" section, follow these steps:
Locate the Attribute GUID value and the Entity GUID value. To do this, start Microsoft Dynamics CRM 4.0, click Settings, click Customization, and then click Customize Entities.
Double-click the entity that contains the inactive attribute.
Double-click the attribute that is inactive.
Press F11 to receive the QueryString URL address. The URL address will resemble the following: