I tried a different login theme (on Fedora 40 KDE spin) and got this. My guess is it means the theme is not compatible with plasma 6, but can anyone confirm or identify the real problem?
I confirm that the error message says … please use an other theme 
F40 is till Plasma 5 right? Use a plasma 5 theme then. Or upgrade to F41 and use Plasma 6 themes.
If I remember correct, Fedora 40 shipped with KDE Plasma 6.0 while Fedora 39 had KDE Plasma 5.
To be honest, I think it says "please install QtQuickControls version 1.1. 
F40 is Plasma 6, but qt5-qtquickcontrols is already installed, presumably part of a qt5 compatibility layer, so that isn’t the answer. However, it says the version is 5.15.15, release 1.fc40. So maybe the issue is that I don’t have version 1.1? Or maybe I just don’t interpret it correctly.
I had a lot of themes saved from when I was on F39, so I just went back to test them. Most of them are like this. That’s why I wondered if it was simple incompatibility with Plasma 6 rather than a random bad one. After all, the theme library is probably not versioned as the software is. But I figured someone must know for sure?