Just want to try to help out if someone is stuck on one of the mentioned problems. As many of you, me and my clan has has several problems with the game. Now we are all online and playing, but still Ubi needs to fix several ingame bugs, crashes and multiplayer. At least we can play, even if it's not perfect... Hope this can help you guys...
One of our guys could not get into the game. Just received a YETI error. He has a NVIDIA graphics card, and updating the drivers helped. He got it going with the latest release found on NVIDIA page. If that does not work for you, try either the beta driver or the previous verson of drivers.
If you have an AMD/ATI card, we don't have any experience with the YETI error, but trying other drivers for your card might help.
Another guy in the clan could access the game, run updates, etc... but could not find multiplayer games or co-op campaign games. We could add him as friend, but he was never showing status online (even if he was!). We figured out it was AVG antivirus that was blocking him.
The recommendation here is to use Microsoft Security Essentials (use it at your own risk) for protection. It does not interfere with your programs or games. All the guys in my clan is using it now and we don't have any issues (read: the protection is good). Other programs can cause the same problems as well, look for any 3rd party protection programs, even the ones installing directly in your browser. Remember, use a protection program for malware, viruses, torjans etc... but try out different ones and find the one that works best for you. Don't stay unprotected!
Hope that this can help someone out. And please don't flame this post if it doesn't work... We are just trying to help!