The system-settings don’t reflect the whole picture in Kubuntu. Settings can be done via the command-line. Here exemplary to chromium:
gvfs-mime --set x-scheme-handler/http chromium-browser.desktop gvfs-mime --set x-scheme-handler/https chromium-browser.desktop
I had an issue with this for quite some time, I finally found the answer via “opening web link from evolution uses wrong browser”.