Minecraft Launcher supports storing credentials in Gnome Keyring. It seems, however, that it does not support storing passwords in KDE Wallet.

I'm using KDE myself on a Debian-based distributive of Linux. I've encountered the same error as MCL-19840. After reading the support page, which is helpfully linked by the error popup, I realized that only Gnome Keyring is supported. I had to install it Gnome Keyring separately, because it is not installed by default with KDE.

Some users of KDE might realize that, some may not. It would be nice if Minecraft Launcher supported KDE's keyring application too.

Other suggestions

• It would be helpful, if the support page were improved with instructions suitable for most popular desktop environments. That is, since Gnome Keyring is sometimes not installed by default, the support page could mention this dependency.
• The Minecraft.deb package could have declared dependencies or suggestions for the gnome-keyring package.