iMac as monitor - brightness stuck to zero

Hi there,

I'm using an iMac 2013 with target display mode for a Macbook pro 2018.

I have the thunderbolt 3 to thunderbolt 2 adapter, and everything works fine expect...

sometime, at random, the imac screen turn "almost" black. I notice the screen is not off, and if I move something around (from the macbook) I can see a faint shadow of a window. So connection is working. Is seems like the brightness is stuck to zero, for some reason.

I tryed to unplug and replug and it came back to life, but now it's stuck like this without recovering.

Also, i read the speaker should get the imput from mac, but if I select iMac as sound output the sound is completely mute.

Any help?

thanks