I started playing yesterday and already got some achievements. However, when I go back into the game, the achievements that I had already taken appear blocked in the achievements list.

Hey, Bigghu, can you repport this problem on this topic?

https://www.gog.com/forum/general/list_of_broken_gog_achievements

We have a small but very effective group of users that help other with achievements issues. It is very important that you report this issue in the topic above, and if you want to participate more actively in our activities and get information faster, I suggest you join our channel on Discord. It will be great to have your presence!

https://discord.gg/g44CFpU

I'll take a look.