If you use the traverse buttons (next back, etc) in a BO group they only work for the BO type you originally open. For Example:
I view an instance of BO1, then click next to view another instance of BO1, works fine. I then click next gain and an Instance of BO2 should be shown, an error dialogue opens "Form XXX not found in object XXX". If I then close the form, return to the grid and open the form, it will open.
In the example above the Group set to use the 'Default Edit form' so the form should be able to be determine.