Fatal Error or Corrupt Data

Steam

Fatal errors sometimes occur when Helldivers tries to merge the data stored in "%APPDATA%\Arrowhead\Helldivers\saves\" with the one in the Steam Cloud and there is a conflict. 

The local files which you'd want to overwrite are "helldivers_save.sav" and "helldivers_save_user.sav". Make sure to create backups of any files before you replace them.

PlayStation

Steps to delete and restore save data:

1. Delete your corrupted save file then launch the game
2. You will see the intro cinematic and land you in training
3. Use Options button to skip training
4. On title menu, select Play (be sure you are online)
5. All your important stats, xp, unlocks, upgrades will be restored

Be sure to complete at least 1 Planet that is not part of any Assault / Defend (it can be any difficulty including Dive in the Park). This is needed in your save file to get one of the Trophies when the War completes successfully.

You will once again have to go through all the same hints, intros and early dialog that are also only in your local save and not restored by the cloud sync. Most of these are going to be one time, and you can turn off hints in one of the options.