ssh – Problems with sftp in Dolphin / Konqueror

I have a problem logging into a remote server as root in Dolphin.
Situation:
I have an id_rsa Keyfile and I can log in with ssh as a normal user and as root without entering a password.

I can also log in as a normal user with dolphin.
Only when I try to log in as root, Dolphin asks me to enter a password. But that does not work, since login with password is prohibited for root.
When I use fish: // root @ server, it also works with dolphin.
This seems to be an old mistake. Is there any solution?

Host and client are both Ubuntu 18.04 systems.