Multiplayer game aborts on startup

Me and my friend play multiplayer campaigns all the time, and as recently as Sunday 31st May it was working fine.

Now for the past two days neither of us are able to host or resume a campaign with one another. Whoever's computer is last to load in either just closes the window or says 'unspecified error before joining'. This then leads the game to abort immediately for the other player.

Does anyone know of this issue and how to fix?

Thanks in advance