Editing the config.json.persistent worked. But I have the same problem that no apps are shown after login.
Edit:
I created a new sub.domain.tld.json in folder /etc/yunohost/portal
Now I have the same login like at domain.tld. Even public apps are shown.
But I assume if I will make changes in the GUI for domain.tld they will not be applied to sub.domain.tld.