Playing on PCs includes some inherent dangers. There’s always a chance you could end up playing with hackers. Some companies fix their games so the hacks can’t happen. Bandai Namco is taking the approach that anyone with altered save data in Dark Souls III could be banned.
Bandai Namco is checking save data for players. If any exploits or save data modifications are detected on your account, you’ll get an “Invalid Game Data” or “You have been Penalized” notification. This can even happen if someone who is hacking Dark Souls III gives you a gift.
The only way to save yourself is to back up your Dark Souls III save data. You can find it in C:UsersYour-usernameAppDataRoamingDarkSoulsIII. Maybe even keep two backup files, in case your most recent one has been flagged. Replacing it with your old data will unban your account. You may lose progress, but at least you can still play!
Source: MCV