Item state not visible in MainUI

I’m running openHAB3.0.2 on a Raspberry Pi 3B+ (upgraded from OH2.5.12).

My IKEA TRADFRI items have been recreated through the UI this time.

I can’t get the UI to reflect the light bulb’s state. All sliders are to the left. In custom widgets, the state is always returned as "-".

Sounds familiar to you? How can this be solved?

Solved: see: