I Have a Fireface 800 since 1 year now, and I'm very happy with it but...

In January, I sent it back for a Hiss issue on the first input, the Instrument Input, in the front. It was making low frequency hiss, and no sound at all.
I received it back 2 months ago, and now, I have the exact same problem on the 7th input, the preamp, on the front.

Suddenly, no more sound, a big hiss, and the green "Signal" LED always ON.

I don't understand... Is it possible my entire Fireface has a problem ?
Have you ever seen this problem before ?

Thank you a lot,

Simon