Is this when loading a save, loading screens on act transitions, or when another person tries joining?

Are you using any mods?

If you create a new profile and start a new game to test, do you have any problem with another player joining, saving loading or getting to Fort Joy?

There is a known issue with some mods disabled that can cause the game to hang at act transitions, though this also affects single player games. If you had mods enabled previously that are disabled, try re-enabling them temporarily to see if that will get the next act to load. This should be fixed in a future update.

There have also been a few reports of loading saves hanging in multiplayer, but it would finish loading if the client system(s) disconnected.