I have completed several levels and got all experience in some of them, but my achievements won't unlock. :(

The game file is freshly downloaded (4 days ago or so), the .exe is the 2.0.0.5 one. Is there something I'm missing or is it a bug?

i was wondering the same thing. In-game achievements are not unlocking for me at all. I spent almost an hour trying to get the Surfboard Master achievement, and I could've sworn i stayed on the plank for almost 6 seconds, yet no achievement. None of the other achievements that are progress based are unlocking as well.

Why is this happening?

EDIT:
After a bit of research it looks like there was an update to build 434 that fixed this issue.

HOWEVER the latest version available on GOG (installer 2.0.0.5) is only build 425, which is probably why the achievements are bugged. Are there any plans to update the GOG files?