$ dolphin
kf5.kio.core: Refilling KProtocolInfoFactory cache in the hope to find "stash"
qt.accessibility.core: Cannot create accessible child interface for object:  PlacesView(0x5626a0827f30)  index:  14
kf5.kio.core: couldn't create slave: "Meldung von klauncher: Fehler beim Laden von /usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kio/file.so"
kf5.kio.core: couldn't create slave: "Meldung von klauncher: Fehler beim Laden von /usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kio/file.so"
kf5.kio.core: couldn't create slave: "Meldung von klauncher: Fehler beim Laden von /usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kio/file.so"
...

When I start dolhin, I see a red message that the creation of the input/output modul is not possible. klauncher said: Error loading /usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kio/file.so

The proposed solution in the thread fixes the issue for me.
https://bbs.archlinux.org/viewtopic.php?id=206352

$ dbus-launch dolphin

About same problem after upgrading from Debian 9.9 to 10.2, when
launching application as ROOT, like Dolphin, Krusader, ...
It's impossible to copy, move, ... anything

ii  kio            5.54.1-1     amd64        resource and network access
abstraction


16:32:45.953-warning default unknown@0 # QStandardPaths: wrong ownership
on runtime directory /run/user/1000, 1000 instead of 0
16:32:46.155-warning default unknown@0 # Qt: Session management error:
None of the authentication protocols specified are supported
16:32:46.167-debug default unknown@0 # System icon theme: "oxygen"
Cannot connect to the D-BUS session bus.
To start it, run:
     eval `dbus-launch --auto-syntax`
16:32:46.355-warning default unknown@0 # QWidget::insertAction: Attempt
to insert null action
16:32:46.357-warning default unknown@0 # QWidget::insertAction: Attempt
to insert null action
DBus Error: org.freedesktop.DBus.Error.Disconnected, Not connected to
D-Bus server
DBus Error: org.freedesktop.DBus.Error.Disconnected, Not connected to
D-Bus server
DBus Error: org.freedesktop.DBus.Error.Disconnected, Not connected to
D-Bus server
DBus Error: org.freedesktop.DBus.Error.Disconnected, Not connected to
D-Bus server
16:35:00.510-critical default unknown@0 # Couldn't start kuiserver from
org.kde.kuiserver.service:
QDBusError("org.freedesktop.DBus.Error.Disconnected", "Not connected to
D-Bus server")
16:35:00.542-warning default unknown@0 # QStandardPaths: wrong ownership
on runtime directory /run/user/1000, 1000 instead of 0
16:35:00.542-warning kf5.kio.core unknown@0 # KIO Connection server not
listening, could not connect
16:35:00.542-warning kf5.kio.core unknown@0 # couldn't create slave:
"Impossibile creare un socket per eseguire un io-slave per il protocollo
«file»."
16:35:00.543-warning default unknown@0 # QStandardPaths: wrong ownership
on runtime directory /run/user/1000, 1000 instead of 0
16:35:00.543-warning kf5.kio.core unknown@0 # KIO Connection server not
listening, could not connect
16:35:00.543-warning kf5.kio.core unknown@0 # couldn't create slave:
"Impossibile creare un socket per eseguire un io-slave per il protocollo
«file»."


Launching dbus-dolphin

dbus[6973]: Unable to set up transient service directory:
XDG_RUNTIME_DIR "/run/user/1000" is owned by uid 1000, not our uid 0
QStandardPaths: wrong ownership on runtime directory /run/user/1000,
1000 instead of 0
Qt: Session management error: None of the authentication protocols
specified are supported
kf5.kservice.services: The desktop entry file
"/usr/share/applications/org.gnome.ChromeGnomeShell.desktop" has Type=
"Application" but no Exec line
kf5.kservice.sycoca: Invalid Service :
"/usr/share/applications/org.gnome.ChromeGnomeShell.desktop"
QStandardPaths: wrong ownership on runtime directory /run/user/1000,
1000 instead of 0
kf5.kio.core: KIO Connection server not listening, could not connect
kf5.kio.core: couldn't create slave: "Impossibile creare un socket per
eseguire un io-slave per il protocollo «trash»."
kf5.kio.core: "Impossibile creare un io-slave. Impossibile creare un
socket per eseguire un io-slave per il protocollo «trash»."
QStandardPaths: wrong ownership on runtime directory /run/user/1000,
1000 instead of 0
kf5.kio.core: KIO Connection server not listening, could not connect
kf5.kio.core: couldn't create slave: "Impossibile creare un socket per
eseguire un io-slave per il protocollo «file»."
QStandardPaths: wrong ownership on runtime directory /run/user/1000,
1000 instead of 0
kf5.kio.core: KIO Connection server not listening, could not connect
kf5.kio.core: couldn't create slave: "Impossibile creare un socket per
eseguire un io-slave per il protocollo «file»."