I'm using KDE 5.8.5 and experiencing same problem here. The software
sources dialog never opens!
I'm using Ubuntu 16.04 with Kubuntu-backports ppa.
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Not completely non-lightdm, Unity session can be started using gdm3.
Only sddm has problem starting either GNOME3 or Unity session
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https://bugs.launchpad.net/bugs/1569357
The possible workaround is unsetting `QT_QPA_PLATFORMTHEME` in
~/.profile file. A workaround is described in this askUbuntu answer
written by me http://askubuntu.com/a/817066/61218
You put these in ~/.profile
## Fixing QT style override problem
if [ "x$XDG_SESSION_DESKTOP" == "xKDE" ]; then
The problem is caused by 3rd party application installing own versions
of Qt libraries. In my case, it is MF 710 3G modem used by sold by some
mobile operators here (Bangladesh). The libraries are in
`/opt/Teletalk_3G/usr/lib` (for Teletalk provider). The solution is just
remove those libraries.
