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[MacTron]

Flashing LED Fault Indicator - Apple Cinema Display.
Blinking sequence :
#1 short, short, short-->Wrong video format or an unsupported resolution.
#2 short, long, short -->Issues with the power adapter.
#3 short, short, long -->The display is detecting a backlight error.

Mine is showing #3 

[GaryN]

Sadly, my experience with these is that a "backlight error" is actually an inverter failure. If one CFL backlight goes out, there's no error indication, perhaps because it's pretty obvious one's burned out.

[Protools5LEGuy]

I see the next MARL  summer project. Retrofitting  LED  backlight to an ADC display.

[MacTron]

Sadly, my experience with these is that a "backlight error" is actually an inverter failure. If one CFL backlight goes out, there's no error indication, perhaps because it's pretty obvious one's burned out.

You are right, everything points in to that direction. The monitor works well for about five minutes and then the upper part goes dark.