For a program/project which has any child projects, the gantt chart throws the below error
If one of the child project is corrupted in the WBS level, all the parent projects/programs will throw the error.
Remove the child project/programs and add them one by one to understand the problemetic project/program. Once the project is identified, open the project in MSP/OWB and save it back. This will fix the corruption and then the parent project/program will open up successfully in gantt.