UPDATE 1: I've managed to contact the developers on discord and received a response: "Seems like a bug, we'll look into it".

UPDATE 2: The game has been updated and everything seems to be working correctly now.

Has anybody been able to play a fresh new game, and unlock the "Monstro" campaign, after the release of the new DLC campaign?

The detailed story: A couple of days ago, I tried playing Against the Moon for the first time, since the developers had released a new DLC campaign.

I played and completed the "Prologue" campaign, and that should have unlocked the "Monstro" campaign. But on my end it remains locked, as you can see from the attachment. I tried playing with Galaxy, and without, I tried redoing the prologue, I tried starting a new game and finishing the prologue, I tried taking all the alternative paths during the prologue... and still nothing works.

I've posted in the game's dedicated forum, and I even tried contacting the developer account that posts here, but until now... nothing... so maybe I can get some help form the GOG community.

I guess I'll have to be bumping this once or twice. :(

Or you could try posting in the actual subforum for this game where the developer seems to be active.
https://www.gog.com/forum/against_the_moon

EDIT: Nvm, seems like you already did ;) - you could always try to contact the dev through PM here or see if you can find an email adress for support and contact them that way.

Which I also did and mentioned in the OP. The dev account has PM disabled.

I tried visiting the steam forums in order to see if I could find some fix for OP's issue and I was amazed to discover that the game is not available for steam.
It's the first time I see a new indie game not available there 8|

The developers were just as amazed to find out that their games were removed from STEAM. As far as I know, they still haven't gotten a response from STEAM, as to why they got delisted.