Since (unstable) update of qt-5-webengine to package qt5-webengine-5.15.3-2-x86_64.pkg.tar.zst my kmail broke: the message window as well as the preview pane only shows white background, no headers and no message at all. Only the “message source” lets me view it.
Downgrading to 5.15.2 immediately solves the issue.
I did not test konqueror.