Why do I have to finish the prologue to get online if I've already beaten the game?

I have already beaten GTA 5, and it was working fine. I was able to play online with friends without any difficulty. A random error while joining a session caused me to go back to solo play, and when I attempted to go online from the pause menu, it said I have to finish the prologue.

I have already beaten the game, so I'm not certain why I should I have to finish the prologue. Can anyone explain this, and is there a solution to this? I would really like to be able to play online, again.

Solution 1:

When this happens, you should manually load the save file where you have completed the game, and then attempt to reconnect to multiplayer. This should solve the problem.