So, me and my friend are having the same problem as you all and we've been watching this thread like hawks for we are very anxious to play this supposedly great coop experience. Now just like you we've been trying every possible way to work this problem out that we could come up with to the point where we were trying out rather ridiculous theories and far fetched ideas just in the hope that one of them would pan out.
Now the thing we noticed today is that every person that we can join and seems to be able to actually play a game in on-line coop, as proven by their lobby having 2/2 slots, is playing the game in French. This can be seen from the map. Most of them are playing on a map called Cys(é)al, the starting area, rather than the English Cyseal and another is playing a map called Bois de Luculla which I suppose would translate into something like forest of Luculla.
The question rises, is this a coincidence or have others noticed this as well, fellow players if you have noticed please leave a comment confirming this or pay attention to this when you are trying to get the game going next time. If there is some truth to this theory maybe it is something worth looking into by the devs as to where the problem lies and perhaps how it can be resolved.

As a final note to Raze and other moderators/Dev people reading this please note how I am calm and polite in this here post of mine, even hoping to be proposing some sort of solution and helping everyone out, despite getting a mail from your support center treating me like not getting the coop to work must mean I have feces for a brain (Seriously asking a person whether they have started a game in multiplayer rather than single player when trying to play coop is borderline insulting) for not a word requiring censorship was used. However if this issue is not addressed in some sort of satisfactory way within the next couple of days this will be the last title I will be purchasing from Larian studios and I will recommend to others to refrain as well using stronger language than I did here.