co op

So my friend and I have been trying to finish off the Greek Revolt quest line but he keeps getting this error as soon as we push towards the top side of the map and we're not sure why its happening. He's done all the trouble shooting he could do to try and fix the problem but its still...

hey! me and my m8 are trying to play co op but when we try it just saying it strating a game then crash anyone know how to fix it?