Heya. This problem is caused most likely by the new steam release. The netcode seems to be not optimal in the current 2.700 patch and will hopefully fixed/improved by relic in the future. Steps that may help you are:
1) Turn off your windows firewall, you can do that by clicking "start -> control panel -> firewall". After you finished playing the game just enable it again.
2) open the following ports in your router via
port forwarding3478, 4379-4380, 6113, 3478, 27000-27050 all TCP and UDP.
If you're a bit lucky you'll be able to play. By doing that I managed to get into ~ 75% more games compared to before, and most people can join when I host a match.
Greetings