I downloaded a sample project from internet which uses ICustomTypeProvider to load the data to a DataGrid and added the Telerik RadComboBox and it seems to work as expected. Attached I am sending you this project. If you could modify it, so that the issue can be reproduced and send it back to us, it will be really helpful for tracking down the source of the problem.
Looking forward to your reply.
Kind regards,
Konstantina
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>