New results:
Switched from simple wall wart to stabilized, way more expensive wall wart with adjustable voltage. 
Tried voltages from 3V (no show) to 9V.
Matrix had at least one failing line with all voltages. 
Best,
Jens
New results:
Switched from simple wall wart to stabilized, way more expensive wall wart with adjustable voltage.
Tried voltages from 3V (no show) to 9V.
Matrix had at least one failing line with all voltages.
Best,
Jens