As I already wrote in another thread on this forum, there might be some problems with matchmaking when someone creates a lobby and takes focus away from the game's window on Windows, e.g. switches to another program or locks the screen. This is caused by a bug in Unity that the game is based on that prevents it from using GOG's library as designed.

Currently we are working on a work around for this issue. As for now, please do not switch to other applications during matchmaking on Windows.