For me it looks like the achievements are not working anymore. I got some before and have 14 achievements. The achievments page only showses the achievements I already have and says I have 100% but this can not be true.
Is this a general problem or is it only broken for me? And how could this be fixed?

You're not the only one, it is affecting me too, the problem started around November, it was briefly fixed when the IT Expansion Update was released but the bug has since returned, If you look at your stats page using your Web Browser, it will show the correct Percentage, in addition, the "My Activity" section will have missing Icons and hovering over them will show an Blank Message, It is not possible to see Achievements on the (Now Useless) Overlay, it has been broken since the Forced Update to GOG Galaxy 2.0 Beta.
*Update*
The Problem has suddenly disappeared (For me at least), all the icons for the Achievements are showing, Completion percentage is being shown correctly and all Incomplete Achivements are being Listed, I even had an couple of new Achievements when I started playing again, I will keep an eye on the Achievements page on Galaxy, the problem could re-appear again (Like it did last time)