As i do not own a Mac i cant be 100% certian on this error. As a suggestion if you can find where your saved ship designs are and remove them to another location and run the game again, it might solve the problem.
I just bought galactic campaign on Window XP Pro SP3. Started geting this error on day five. Full stripped and reloaded all from stock installers. Same error for hulls in dlc from forums I did not reinstall. Added missing content, and error switched to new packs. Short story long I had Galactic Concordium and it is dead and gone and the machine wants it. Is there a reg key to yank so I can clean install without the ghost of loads past popping up. I have stripped and uninstalled eight times now and it is a vicious circle, I have purged over 11 gb of data to assure, defraged, disk cleaned and rebooted dozens of time between steps still no dice, HELP.
To complete a clean installation you need to delete 2 locations: (Warning, any progress you might have will be lost)
\My Documents\My Games\GratuitousSpaceBattles\ <- This is where the game holds all your ship designs
\Gratuitous Space Battles\ <- this directory can be anywhere depending on where you installed the game
Once you delete those 2 directories you should be able to reinstall the game with your DLC’s and you should be free of errors.