With the update to Qt 5.9 the qtvirtualkeyboard is broken as it's still Qt 5.7. This breaks e.g. virtual keyboard integration in the lock screen and KWin.
Setting up qtvirtualkeyboard (5.9.0-0neon+16.04+xenial+build15) ... Setting up qml-module-qtquick-virtualkeyboard (5.9.0-0neon+16.04+xenial+build15)