I'm in the same boat. Been doing everything Support has told me to, nada. Have reinstalled everything from Uplay to Far Cry 3, still nothing. My friend can play just fine, but apparently FC3's servers just hate my computer.
I bought Far Cry 3 from Epic Games and playing it in GeForce Now. I can play story mode without any problems but can't play Multiplayer or Co-op as I keep getting the same error message :/