[SmartSarah]

Small update
In order to measure the BL voltages I hooked up a small multi-meter with it's tilt-stand hanging on the power board.
Since then the TV worked normally.
So bad joint close to the upper part of this board? Earlier measurements with the same multi-meter setup gave me a similar impression but after a while the error returned, so I left the theory.
Now, after measuring the BL tensions, hanging the multi-meter on the board and applying some mechanical pressure on the backlight connector in order to make good contact, the error remained away.

So I took the power board and re-soldered all pcb joints in the power lanes starting from the LED+ connector down to the two diodes on the secondary side. This includes a power diode, a transistor, a coil, some resistors and bridges. This for the 3 LED backlight circuits. From experience I'm aware that "hot" power components might de-solder themselves. Some of the joints looked suspicious but I honestly can't tell if they really where troublesome. Since we started using low lead solder, good joints don't shine anymore!
So far, problem gone!!!

Anyway, I'll observe situation for a few weeks before I feel confident I've definitively hit it.
If the problem stays away, I'll call it off. Meantime, many thanks to Diah who gave me adequate directions.

Cheers

Sarah