After having only silence from the support's part when I reported this issue over a year ago, I still have it with my friend. We can't load into battles together - after we load in, we are stuck "waiting for player". But after some testing, we narrowed it down. So it seems we can load in when we have only a few units on the battlefield. When we have each about 20, we get the issue. When it's around 10-15 or less, it seems just fine. This issue happens both in MP skirmishes and MP campaign battles.
We have also tested with various unit sizes and they don't seem to matter. It's not about the model count, it looks like, but about units. It will work with 10 units of skaven slaves, but it won't work with 20 units of giants. We have also changed our computers over that year and reinstalled windows, so it's definately something to do with the specifics of the game or -his- provider (since I don't have that issue with some other players). I've no idea how to figure it out further, but obviously I'd love a fix.