In Ubuntu 20.04.2, Krusader (v. 2.7.2-1build1), many icons are missing.
When launching krusader from the command-line, the errors below are listed:
$ krusader
QCoreApplication::applicationDirPath: Please instantiate the QApplication object first
11:46:31.467-debug default unknown@0 # System icon theme: "Yaru"
11:46:31.515-warning default unknown@0 # QWidget::insertAction: Attempt to insert null action
11:46:31.517-warning default unknown@0 # QWidget::insertAction: Attempt to insert null action
11:46:31.524-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Yaru/16x16/legacy/"
11:46:31.524-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Yaru/22x22/legacy/"
11:46:31.524-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Yaru/24x24/legacy/"
11:46:31.524-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Yaru/32x32/legacy/"
11:46:31.524-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Yaru/48x48/legacy/"
11:46:31.525-warning default unknown@0 # Invalid Context= "Apps" line for icon theme: "/usr/share/icons/Yaru/256x256@2x/apps/"
11:46:31.525-warning default unknown@0 # Invalid Context= "Mimetypes" line for icon theme: "/usr/share/icons/Yaru/256x256@2x/mimetypes/"
11:46:31.525-warning default unknown@0 # Invalid Context= "Mimetypes" line for icon theme: "/usr/share/icons/Yaru/scalable/mimetypes/"
11:46:31.528-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/8x8/legacy/"
11:46:31.528-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/16x16/legacy/"
11:46:31.528-warning default unknown@0 # Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/16x16/ui/"
11:46:31.528-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/22x22/legacy/"
11:46:31.528-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/24x24/legacy/"
11:46:31.528-warning default unknown@0 # Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/24x24/ui/"
11:46:31.528-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/32x32/legacy/"
11:46:31.528-warning default unknown@0 # Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/32x32/ui/"
11:46:31.528-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/48x48/legacy/"
11:46:31.528-warning default unknown@0 # Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/48x48/ui/"
11:46:31.528-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/64x64/legacy/"
11:46:31.528-warning default unknown@0 # Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/64x64/ui/"
11:46:31.529-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/96x96/legacy/"
11:46:31.529-warning default unknown@0 # Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/96x96/ui/"
11:46:31.529-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/256x256/legacy/"
11:46:31.529-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/512x512/legacy/"
11:46:31.529-warning default unknown@0 # Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/scalable/legacy/"
11:46:31.529-warning default unknown@0 # Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/scalable/ui/"
11:46:31.970-warning default unknown@0 # Unable to find icon "chronometer" of size QSize(16, 16) in any configured theme
11:46:31.976-warning default unknown@0 # Unable to find icon "bookmarks" of size QSize(16, 16) in any configured theme
11:46:31.996-warning default unknown@0 # Unable to find icon "arrow-up" of size QSize(16, 16) in any configured theme
11:46:32.068-warning default unknown@0 # Unable to find icon "edit-select-none" of size QSize(22, 22) in any configured theme
11:46:32.079-warning default unknown@0 # Unable to find icon "edit-select" of size QSize(22, 22) in any configured theme
11:46:32.114-warning default unknown@0 # Unable to find icon "user-identity" of size QSize(22, 22) in any configured theme
When going over the menus, more missing icons are listed:
11:48:47.189-warning default unknown@0 # Unable to find icon "edit-rename" of size QSize(16, 16) in any configured theme
11:48:47.211-warning default unknown@0 # Unable to find icon "split" of size QSize(16, 16) in any configured theme
11:48:53.264-warning default unknown@0 # Unable to find icon "edit-select" of size QSize(16, 16) in any configured theme
11:48:53.279-warning default unknown@0 # Unable to find icon "edit-select-none" of size QSize(16, 16) in any configured theme
11:48:53.299-warning default unknown@0 # Unable to find icon "edit-select-invert" of size QSize(16, 16) in any configured theme
11:48:59.202-warning default unknown@0 # Unable to find icon "character-set" of size QSize(16, 16) in any configured theme
11:49:03.296-warning default unknown@0 # Unable to find icon "trash-empty" of size QSize(16, 16) in any configured theme
11:49:07.057-warning default unknown@0 # Unable to find icon "tab-duplicate" of size QSize(16, 16) in any configured theme
11:49:07.071-warning default unknown@0 # Unable to find icon "tab-close" of size QSize(16, 16) in any configured theme
11:49:07.083-warning default unknown@0 # Unable to find icon "user-identity" of size QSize(16, 16) in any configured theme
11:49:07.113-warning default unknown@0 # Unable to find icon "view-split-top-bottom" of size QSize(16, 16) in any configured theme
11:49:13.049-warning default unknown@0 # Unable to find icon "configure" of size QSize(16, 16) in any configured theme
Using apt-file search to look for the missing icons, they seem to be in various *-icon-theme packages.
The breeze-icon-theme package, described as "the default icon theme for the KDE Plasma 5 desktop.", seems to have all the missing icons.
So the fix was
sudo apt install breeze-icon-theme
So the real problem is in the listed dependencies for the krusader package. It should probably be corrected to add this icon theme, possibly with alternatives if other themes also contain all the missing icons.