Me n Wolf n Sparks have all had the same problem. We invite for co-op, other person accepts and gets “Waiting for host to start the game”. the host doesn’t get any recognition the other person has accepted though and the game doesn’t start.
After some stuffing around i found that if you are running Avast! you need to disable your web shield and possibly your p2p shield and you can then enjoy you co-op in bf3 with your friends :laugh:
I hope this helps some people 