COOP unplayable since last update(s)

Ever since the last larger update, co-op has become unplayable. By now it has gotten so bad that co-op players are just unable to enter/load any battle or rouste. For them, a black screen shows, some sound plays and the exit button is shown. This is now the case all the time (used to be every other time), and sometimes even the host is stuck in a black loading screen. We are playing on PC (GOG version)

In sum this means that we just cannot play together, which in turn means we will stop playing this game. This is very frustrating, especially so with a 100-hour savegame.