I've created an account just to share this experience with you guys in the hopes that you can avoid what I just went through. I read this subreddit everyday and can see how dedicated you guys are to the game and wouldn't want this to happen to you. Last night after closing the game application, I did my back up save routine (saved my data on the cloud AND on a USB stick). Too my demise, I woke up this morning to the infamous CE-34878-0 error code. When I boot up the game, my guardian spirit is missing on the menu screen and the game consistently crashes when I try to load my save file.
The look on my face when I realized that I overwrote ALL of my back up saves with a corrupted file would make for a very dramatic piece of photography. I've looked up and attempted every suggested troubleshooting method to fix this and nothing has worked. When all else failed, I even dusted off my original PS4, downloaded Nioh and attempted to load up the game save on THAT system and ran into the same error, which presumably confirms that it's a save data corruption issue. There was nothing unusual when I closed the application last night. The game did not crash and I ALWAYS wait for the save symbol to finish its animation, so I have no clue how my save file got corrupted (and I certainly wish I knew it was before I decided to back everything up).
So how do you avoid losing 110+ hours of gameplay on a level 750 character to boot? Some of you may already know the answer, but in case you don't – back up your saves, and not just on the cloud, but ALSO on a USB stick. Most importantly: DO NOT back up everything all at once. In the future I plan to upload save files on a USB stick separate from when I upload it onto the cloud and then update it sporadically.