I went to System Settings - Common Appearance and Behavior - Locale - Country/Region & Language - Languages. American English was in the Available Languages box. I moved it to the Preferred Languages. I got the message that in order for it to take effect, I need to log out and then back in. I'll let you know if that works.

That leads to the question of why would changing the language fix a permissions problem?

If you look at the *.desktop files you will note that they have many keys in them in the form of:

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Name=foo
Name[en_GB]=foo
Name[en_NZ]=foo

The en_GB/en_NZ are language codes, which will be preferred by KDE's configuration system if present. In your case, you were using the default language, en_US - which doesn't have it's code specified in the configuration file usually. For unknown reasons it was being specified.

Thank you for the help. I went through the .desktop files and noticed that some of them had 2 entries for Icon=. I removed the one that had [EN_us]. I am now able to change the icon through the Properties like I should.

Any ideas as to what caused it? Should a bug report be opened? If so, to what component? Who should open it?

Again thanks for the help.

Unfortunately I don't know why this occurred. One guess is that the en_US code was somehow specified in a KDE config file (likely kdeglobals) instead of simply not being set.

It is likely worth filing a bug report (against kdelibs, namely kconfig/kdesktopfile) as this is definitely a bug in kdelibs.

This is pretty old but I got this error too, even in 2019. I noticed in the desktop file are some values localized. It seems problem comes when Icon is also localized.
When you change:

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Icon[en_US]=gnome-panel-launcher

to

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Icon=gnome-panel-launcher

it just works. Problem might be that I use both Gnome and KDE and desktop file was created in Gnome (which supports Icon localization).