I recently rebuilt my Qt, KF5, and Plasma 5 environments from scratch. I noticed that System Settings no longer started.

It took me a while to figure out the error message but the proximate issue ended up being:

file:///home/kde-svn/kde-5/lib64/qml/org/kde/kirigami.2/templates/private/ScrollView.qml:95:9: 
"Connections.enabled" is not available in QtQuick 2.5. 
             enabled: !Settings.tabletMode 
             ^

I imagine that Qt 5 has stricter checking on the use of QML properties. In this case we ask for QtQuick as of Qt 5.5, but Connections.enabled is only supported in Qt 5.7 on.

I fixed this (and one other such usage) by changing the import statement to ask for QtQuick 5.7. We already require at least Qt 5.7 anyways so this isn't a new dependency for current KF5.

Installed the updated Kirigami, successfully launched System Settings. Additionally the Plasma desktop wallpaper started working again too, though that wasn't why I'd made this fix.

I verified by manual inspection of all uses of the 'Connection' QML item that there were no other uses of the "enabled" property from Qt 5.7.