So, I have activated encryption (as well as defined the master password). But I think it's not working, because if I open the database.sqlite with a text editor I am able to see all its data.
Well, maybe I don´t know really how to set it properly. But I can see inside Joplin, at Options ==> Encryption, that there are currently 470/470 items decrypted/not encrypted. I know that I am probably missing something here, but can't figure what I am doing wrong.
You are not doing anything wrong it's just you have misunderstood what Joplin does when End to End Encryption (E2EE) is enabled.
When activated Joplin encrypts your notes and attachments when they leave your computer and whilst stored on whatever service you are using to sync your Joplin clients. It prevents whoever runs your cloud storage service from being able to see, search, index your notes. Also if that account or storage service is compromised your note data is still protected.
Think of each client you have as an "end". From the point your new or updated notes leave one client to the point they reach another of your clients, via the cloud service, they are encrypted.
The note data and attachments are not encrypted on your computer. This is the Joplin FAQ entry regarding why Joplin does not have local encryption or a passcode / PIN lock.