This can be a problem with the Main UI. I have tested with the latest snapshot release (2145) and the expert mode works for me. There have been some fixes regarding item definitions read from items files.
Do you see any warnings or error messages in openhab.log.
But I did not try to import an items file. Can you try the latest snapshot (please clear the browser cache after the update)? If this does not help please post the item definitions for the device that are causing this problem. Maybe there is a definition in the imported items file that causes a problem.