Can't join co-op career session/lobby

I don't think this will be much comfort to you, but I've seen that error message before, once or twice in the past, but it's only occurred for us temporarily, and we've been able to wait a couple of minutes for it to clear before playing without issues. I assume in that moment that it probably _was_ the servers not responding. But it sounds like you've got something more serious going on. So all I can say is 'good luck' and I hope you find a solution (I know that would drive me nuts).