I tested it now for myself, for some reason ProFTPD only allows the owner, who owns the directory, to upload to it. Root can not upload to a folder he does not own, which is weird IMO.
So I did:
adduser ftpuser
usermod -a -G www-data ftpuser
Then connecting with this user works.
Does somebody know why ProFTP behaves like that? With samba I don’t have this issues.