"It does not allow me to configure the keyboard shortcut only with the Windows key for the application launcher" KDE Plasma Version: 5.25.5 KDE Frameworks 5.9. 5.15.6 QT
You don't need to; In Plasma 5.26 the Application Launcher always opens with the Windows key (unless you used Latte Dock in the past, which overrode this). In Plasma 5.27 we plan to actually allow setting modifier-only keyboard shortcuts (e.g. the just the windows key), but that isn't necessary to fix the bug you're experiencing. For now, since you're still on Plasma 5.25, you should be able to fix the bug you're encountering by setting "Alt+F1" as the keyboard shortcut to activate Application Launcher widget.