After an upgrade from 42.3 to 15 i had exactly the same error with kdesu hangs and “no protocol specified”.
The solution, at last for me, was to create a new user. After booting with this user everything was back to normal again.
Comparing the new users /home with the old one there were some differences.
In my case i simply deleteted the folders kde, kde3 and dbus in /home of the old user, rebooted and logged in my old user.
Everything is working again flawlessly with my old user now.
I hope this helps.