Icon start to be missing in the Ubuntu 20.04 (Ubuntu 18.04 was fine). Maybe this output is related:
$ kdiff3
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/8x8/legacy/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/16x16/legacy/"
Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/16x16/ui/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/22x22/legacy/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/24x24/legacy/"
Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/24x24/ui/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/32x32/legacy/"
Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/32x32/ui/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/48x48/legacy/"
Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/48x48/ui/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/64x64/legacy/"
Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/64x64/ui/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/96x96/legacy/"
Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/96x96/ui/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/256x256/legacy/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/512x512/legacy/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/icons/Adwaita/scalable/legacy/"
Invalid Context= "UI" line for icon theme: "/usr/share/icons/Adwaita/scalable/ui/"
Icon start to be missing in the Ubuntu 20.04 (Ubuntu 18.04 was fine). Maybe this output is related:
$ kdiff3 icons/Adwaita/ 8x8/legacy/ " icons/Adwaita/ 16x16/legacy/ " icons/Adwaita/ 16x16/ui/ " icons/Adwaita/ 22x22/legacy/ " icons/Adwaita/ 24x24/legacy/ " icons/Adwaita/ 24x24/ui/ " icons/Adwaita/ 32x32/legacy/ " icons/Adwaita/ 32x32/ui/ " icons/Adwaita/ 48x48/legacy/ " icons/Adwaita/ 48x48/ui/ " icons/Adwaita/ 64x64/legacy/ " icons/Adwaita/ 64x64/ui/ " icons/Adwaita/ 96x96/legacy/ " icons/Adwaita/ 96x96/ui/ " icons/Adwaita/ 256x256/ legacy/ " icons/Adwaita/ 512x512/ legacy/ " icons/Adwaita/ scalable/ legacy/ " icons/Adwaita/ scalable/ ui/"
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "UI" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "UI" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "UI" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "UI" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "UI" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "UI" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "Legacy" line for icon theme: "/usr/share/
Invalid Context= "UI" line for icon theme: "/usr/share/
All that folders exists