Ok, now I got the vault synced on the phone. I would suggest renaming the “Create new vault” button or make a separate onboarding process for syncing to an existing vault, but that’s just my opinion.
However, now a new problem occurred. After syncing on the phone, making changes to the files on the mac gives an error message every time, like this:

When I open a file in the mobile app and edit it, the changes is synced and visible on the mac after a few seconds. But if I then try to edit the file on the mac, the error message appear, and the changes done on the mac is not visible on the phone.