iPad boots, after Apple logo backlight goes off

Dear iFixit-Friends,

my ipad starts normal and shows apple logo when booting. After a few seconds the backlight goes off.

Tried another mainboard in the same housing with the same LCD and digitizer, this is working perfecly.

While restoring via DFU or recovery the LCD shows normal image and backlight. After restore without problems the ipad starts again and shows apple logo with the installation bar below. After 1/3 in progress the apple dissapears and backlight goes off while the device boots completly, so that it is normally detected in 3uTools without any problems and it makes sounds when charger is plugged in.

Where to search on the mainboard? (we have equipment for microsoldering and measuring)

Any idea or solution?

Thanks a lot and greetings MB