Thanks, installing libqt5libqgtk2 brought most of the icons back. Not all of them, but at least the important ones.
E.g. in dolphin, the icons in the places bar for "Root", all of the "Recently saved" entries and in the "Search for" part the icons for "Documents", "Images" and "Audio Files" are still missing. For them, dolphin still reports the "QPixmap::scaled: Pixmap is a null pixmap" message.
Thanks, installing libqt5libqgtk2 brought most of the icons back. Not all of them, but at least the important ones.
E.g. in dolphin, the icons in the places bar for "Root", all of the "Recently saved" entries and in the "Search for" part the icons for "Documents", "Images" and "Audio Files" are still missing. For them, dolphin still reports the "QPixmap::scaled: Pixmap is a null pixmap" message.