As per always, after a update, servers will be down a few minutes and when they open, they will let people in in waves. If it is opened completely, it means 120x3 people downloading the mission files which means RIP servers. So be patient, spam the refresh button and jump in as soon as it is opened. Or do as I do and come back a few hours after a update so you don't have to join the masses who are downloading it simultaneously.