After finishing the first mission, I can't continue. Choosing "Mission Briefing" does nothing. Attached is an image of the screen I'm on.

There seems to be no proper solution for this. Others loaded a saved game and finished the mission again until they were able to proceed. You should save near the end of each mission, just in case the issue appears again.

Of course you could try running the game in compatibility mode for Win XP SP3 and explicitly as an Administrator (right-click => Run as administrator). That might help or not, but it's always worth a try.

Seriously? I just purchased this on GoG.com because a review on the front page explicitly stated it fixed the issues from the Steam version.

Stop selling the damn thing if people can't get past the first mission without trial and error! What a horrendous waste of time.

Have you bothered to tell GOG about this problem?

This happened to me the first time.

I replayed the mission, saving a few times along the way just in case and I was able to level up the characters and proceed.

Happened to me as well.

Weird bug. Not a massive problem (bit annoying if you forgot to save during mission 1, but even then you can skip the tutorial and follow the red arrows, so not too bad). Reloaded a save from middle of mission 1 and it was fine.

I did wonder whether it had something to do with "save"ing as soon as the mission completed, which I did do the first time, but not the second.

Five years later and it's still not fixed. I know, it's not a total game breaker, but come on.