I'm afraid that the provided information is not sufficient in order to trace the exact cause for the described behavior. However can you please verify that you are not repopulating RadGrid control before you are trying to access checkboxes value.
All the best,
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking system and vote to affect the priority of the items.