I don't know much about this game but if I were you, I would probably try the following ... Right click on the save folder in the SAOFB folder in appdata and go to properties and then security, click edit, then add. Then type everyone and press enter. Then click on everyone and check every box under the allow category then press apply. I figure this will rule out any sort of issue with file permissions. And its not like there's any sensitive information in there anyway. Worse case scenario, a hacker steals a copy of your save file :P

Then I'd run the program as an administrator. You know how to do that right? Go to your game directory. Steamapps>common>Sword art online fatal bullet > SAOFB > Binaries> Win64. Right click on the exe file and select run as administrator. It might work doing it from the launcher too, idk.

Anyway... I would try those. If no.... there are logs you can check. both windows and the game keep logs, see if you can get any info from them. The game logs are in the SAOFB folder in appdata. And for windows logs you want the 'event viewer' just search for it next to your start menu icon. See if you can find anything that matches up to the exact time this occurs.

Then again, it should be saving to the cloud as well, (unless you have that disabled?). I'm not sure which happens first or if they happen at the same time, I would guess locally happens first, in which case I would think it wouldn't matter. But if you've no luck up to this point, might as well try I guess.

Could also be your firewall. Add an exception for steam and the game, both ingoing and outgoing. Do you have any third party anti-virus software? Try disabling it. Or just try turning off cloud saving.

Well thats all I can think of, short of reinstalling windows. But don't forgot to check those logs, they may hold a clue.

Maybe somebody more acquainted with the game will know more.

Btw... whats your specs?