The text box on the lower right side of the screen on startup claims that the game is not the latest version. This also happens if you try to "authenticate". Is there something wrong with the GOG version? Did the devs abandon this? I'm confused.
Maybe there is a fan patch to apply?

I have the same issue and at the same time my app gor fallen London has also stopped working. Is there a connection?

Today Steam has just updated the game and expansion with a new version. Hopefully it will fix that bug. I am pretty sure that this issue is intentional or a consequence of the upcoming changes in the version of the game. If it is the case and the way of doing things, it is very inconvenient.