Hi Jonathan,
Is this happening on a customized screen or the stock CRM_EditCustomer screen? Are you able to test this grid on the stock CRM_EditCustomer screen (By navigating to the Customize tab > Screens and and then click on View beside the CRM_EditCustomer screen) for me and see if the Grid is not loading?
If the stock screen is functioning normally, would you be able to give me a little more information on your custom grid? What fields is it displaying on the back-end? are there any SQL Overrides on the Columns?
Thanks Jonathan.
-Ben