I would suggest submitting a support ticket to PTC for traceback errors.
Other suggestions is to try opening the parts and sub assemblies to see if you can find something corrupted. If you are using Windchill, try adding the assembly to a new workspace, sometimes workspaces corrupt.
Check error message if you find in message area. Many times we ignore messages shown there.
If you don't find any error message, better to log a case into PTC suuport log. Only PTC team can investigate these traceback files.