A blue Light,...what does it do? It lights blue...(or in my case it dont light)

I have 3 CEM+ Dimmerracks each with 96 Channels.
Normaly i switch them on, hear the fans starts, wait 30-50 seconds and the blue "triangle" in the left upper corner lights.
But now, one rack dont do it.
It works normal, the Processor and all Dimmers are seams okay. No Problems.
Only the blue light dont light blue, it is off ( the three phase control orange lamps behind the door upper right corner are all three on)

One day i know for what the blue trinangle was build in.
But i forgoten it.

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