Hey, so I discovered a little workaround to fix the following problem:
-You and your partner can play with random players when searching for coop
-You and your partner can join multiplayer games
-You and your partner can NOT connect to eachother to play coop
What is the actual problem?
For less informed on the topic of networking, there are 2 Internet Protocol versions mainly in use right now: IPv4 and IPv6. IPv4 IPs look like this: 216.98.48.35(which most of you should be familiar with), while IPv6 looks like this: 2a02:908:df53:de20:e515:2eab:1bf8:agf6. It should be clear to see now that IPv4 offers a very limited amount of different IPs as opposed to IPv6.
Currently the era of IPv4 is coming to an end as we come closer to the end of available IPv4 IPs, ISPs(Internet Service Providers) have started to roll out a temporary system known as DS Lite(or similiar). Basiclly instead of having both an IPv4 and IPv6 IP you now only have an IPv6 IP to identify yourself, the IPv4 of your network is shared with a lot of other customers. Having to deal with this causes a lot of problems for users and service providers, especially peer to peer connection between a DS Lite IPv6 and a IPv4 only user are hard to accomplish.
What you can do
Both of you should first of all check if you have this problem by going to a website that checks your IP version(e.g. http://whatismyv6.com/). The easiest way to fix this problem is to use a VPN(despite the fact that ubisoft customer support keeps telling us that those are causing the problem). Most VPNs will change your IPv6 IP to a IPv4 IP. While I can't help you on the choice of VPN provider as I havn't tried too many, you could try some trial versions and see if it's worth for you to purchase one for $5-$10. Personally I'm using OpenVPN right now and the speed and everything seems to be fine, but of course there could be better providers out there.
As soon as you have a VPN, activate it and play. Make sure to deactive it after you are done playing.
What UBI should do
Of course I'm not going to give you a detailed list of fixes, because normally your networking engineers should be well aware of this issue and how to properly setup a working peer to peer connection. Would be great if one of the many community managers at Ubi would be able to get one of the network programmers to comment on this issue here and maybe explain why you do, what you do.
If you don't want to do anything, the amount of IPv6 only customers is rising daily and you will be on the losing end if you keep developing good games, but mess up the networking part of it.
Thanks for reading and I hope I helped some of you.
I've been having this exact same problem with Assassin's Creed: Unity. I've been searching mad for weeks for any answers, even contacted support which didn't really help anything.
Last thing I figure what could cause a problem was the fact my ISPN started using IPv6 a while back. Curiously enough I came across this post.
If this really is what is causing the problem, Ubisoft needs to fix their netcode and not force us to use VPN's because of their bad lousy coding. Keep up with the times Ubisoft, srsly....
Also I'm having this problem on my Xbox One. So using a VPN for my console will be a hassle. Because that would mean that I'll have to connect my xbox trough my PC to be able to get it on said VPN...Not to mention the troubles Xbox live can get in with using said VPN.