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Ok, after removing UE501 (there was some corrosion on it I didn't see) the 1V25 rails are normal and I got 20V, 0.22A and stable PPBUS_AON on DC power.
Forcing th board in DFU worked. Maybe the keyboard is defective. But unforunately I can neither restore nor repair. It throws an "No such directory or file (NSPOSIXErrorDomain)" error...