I can't enter my serial key for multiplayer in M&B: With Fire & Sword. There is an old thread on this game's forum where another user had the exact same issue (For some reason I can't post the link, just search for 'multiplayer' and find the topic "Can't join multiplayer game"), which was supposedly fixed by a patch. I have the game updated to the latest version, but I still have a blank screen where I'm supposed to enter the key.

Is this a common bug with the GOG version?

[Solution found, check my other topic "Quick guide to solving the multiplayer key issue for M&B: With Fire & Sword"]