After removing KDE and switching to xfce, I discovered that Kate has lost all button icons, while Kile has not. I guess some configuration file refers to missing files, but what file is it. I tried to create a new, clean, user account, but the problem remains. In particular the search buttons has disappeared, since they have no text on them.

Update

A fresh install of Xubuntu on a second drive gives the same error:

It is the same machine, booted from an external USB hard drive

The "Open" dialog box is not affected. Only toolbars and the "Save as" dialog are broken. I suspect a packaging bug.

Seems to be a bug. Check here (answers #10 and #16): https://bugs.launchpad.net/ubuntu/+source/dolphin/+bug/1509562

In short: installing package 'libqt5libqgtk2' before 16.04 and 'qt5-style-plugins' for 16.04 and later brings back MOST of the icons in KDE-Applications. For the few remaining ones, you have to fiddle around with the icon-settings in KDE-System-Settings.

HTH

Placing .rcc file into ~/.local/share/kate seems to bring the icons back (this also applies for krusader).

I did what is described in following webpages. However, kate is not using correct icon theme now.

https://dev.webonomic.nl/using-krusader-with-ubuntu-18-04

http://divideoverflow.com/2017/05/Configuring_Krusader_to_use_different_icons_template/

You should install oxygen icon pack:

sudo apt-get install oxygen-icon-theme*

and if the left side panel (file list..) is missing:

sudo apt-get install kdelibs-bin kdelibs5-data kdelibs5-plugins