There's an "Additional Info" panel on the log for that error. Can you provide that information?
Also, what was mapped for the drop down control? Are there variable components in the properties (ID's, etc) that change from run to run? It might have mapped fine the first time but might need to be adjusted.
Based on the error you get, TestComplete cannot find the object. It means that mapping criteria have been changed since the time you had created the test. It's worth checking the criteria you use to make sure that they are reliable (contain IDs, names, etc.).