Unable to create io-slave. klauncher said: Error loading '/usr/lib64/qt5/plugins/kf5/kio/file.so'

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Error within dolphin or konqueror:

Unable to create io-slave. klauncher said: Error loading '/usr/lib64/qt5/plugins/kf5/kio/file.so'

First of all, check that the said library does exist:

$ ls /usr/lib64/qt5/plugins/kf5/kio/file.so
/usr/lib64/qt5/plugins/kf5/kio/file.so

If it does not, then investigate further...

If it does, it is a benign, easily recoverable error.
It happens because Qt (dolphin, konqueror,) loads this plugin and holds in in RAM, but if you do a system upgrade in parallel, then the file handle previously held by Qt is no longer valid.

Solutions:
launch your application using dbus-launch thus:

$ dbus-launch dolphin
// or
$ dbus-launch konqueror

A more permanent solution would be to log out from the current KDE/Plasma session and re-login. Reboot the computer if necessary.