Save File Import

For me, it works this way:

You have 2 files for 1 savegame, for example: SGTA50000 SGTA50000.bak

You need both of them in the folder (C:\Users\USERNAME\Documents\Rockstar Games\GTA V\Profiles\SC_NUMBER). Try deleting all older savegames.

Restart the game and click on "Story Mode", the save will load automatically.

If this doesn't work:

1. Backup the "Rockstar Games" folder (C:\Users\USERNAME\Documents\Rockstar Games\)
2. Delete the whole Rockstar Games folder (C:\Users\USERNAME\Documents\Rockstar Games\) => This is safe, I also did that. But your game settings will be gone if you don't back it up.
3. Restart the game, the folder will be re-created new with all the needed data.
4. Put your newest savegame-files into the folder (both files): C:\Users\USERNAME\Documents\Rockstar Games\GTA V\Profiles\SC_NUMBER\
5. Restart and try again.

Trudler is right, but for Steam version they are at:

C:\ProgramData\Socialclub\YOUR_SOCIAL_ID\NUMBER