[SPOILER]

The last achievement called "Etherborn Complete", doesn't unlock when I complete the game in both modes.
I believe this is a bug, because no one is apparently unlock it in this GOG version.
Please devs take a look at this.
Thanks.

Yeah, I have the same problem. Completing the last level and reaching the end on both modes a second time doesn't help either. I also tried to complete the ending on a savegame I hadn't done it in plus mode before - no dice.

According to the game stats, some people have unlocked it though. I wonder if it was done in a legit way or via some unlocking software.

Also, just in case anyone else encounters this problem: Not a single achievement unlocked on my first playthrough in normal mode. That's because I denied the game access to the internet before, they started working after I granted it. Needless to say, this didn't solve the problem with the final achievement.