An error occurred while calling the callback: "HrGroupGetVisible”

Every now and then, some CRM users around the office complain about seeing the following error message when working with Microsoft Office Outlook where Microsoft Dynamics CRM 4.0 Outlook client is installed.

Custom UI Runtime Error in Microsoft Dynamics CRM: An error occurred while calling the callback: “HrGroupGetVisible” . 

After a few research and goggling, I found this KB article seems to be quite helpful.

http://support.microsoft.com/kb/2494581 

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require you to reinstall the operating system. Microsoft cannot guarantee that these problems can be resolved. Change the registry at your own risk.
To resolve this problem, follow these steps:

1. Exit Outlook.
2. Click Start, click Run, type regedit, and then click OK.
3. Locate the following registry subkey:

   HKEY_CLASSES_ROOT\TypeLib\{2DF8D04C-5BFA-101B-BDE5-00AA0044DE52}

4. Right-click the 2.4 registry key, and then select Export. Save the export to your desktop to create a backup.
5. Right-click the 2.4 key again, and then select delete.
6. Start Outlook.

This same KB is also relevant to Microsoft Dynamics CRM 2011 Outlook client for a similar error message.

Custom UI Runtime Error in Microsoft Dynamics CRM:  An error occurred while calling the callback: "HrExplorerGroupGetVisible"

Hope this helps you, too!