Please look again in the “Einstellungen” of your android client. There you should find the place to set up your E2EE (“Ende zu Ende Verschlüsselung starten”) and where you get your 12 words pasphrase.
Once you created your pasphrase, you can start using E2EE:
Because encrypted folders can cause realy weird side effects in the web client if they are hosted in the root folder, you should create a new folder, lets say E2EE.
Inside of that Folder E2EE you now can create encrypted folders. After such a folder got created, you can choose via the three dots to mark that folder as encrypted (“Als verschlüsselt festlegen”). If a folder is marked as encrypted all content inside of that folder and sub-folders are considered to be encrypted, no need to mark sub-folders.
Please keep in mind: to ensure that the content to be encrypted never comes unencrypted on the server, a folder can only be marked as encrypted, as long as it is still empty. So first mark the folder, then add content.
Only after your E2EE has been set up in the Android client will you be given the opportunity to enter the passphrase in the desktop client.