Hi my Duet suddenly won't boot anymore.

I have measured the power supply it delivers 9 V.
I can see a red light in the optical out socket - and sometimes the front led show a constant red.

I felt that the power connection is a little wacky, therefore I disassembled the Duet and when measuring on the power connection on the backside of the PCB I only measure 5.20 V - is that normal?