Quote - What are your settings? Try 100 for sensitivity, 1/125 for exposure and 1.4 for f-stop. If it's still black, the lights in your scene are not bright enough.

I use the default settings of Lux, I set nothing in lxs.

Quote - s for the fireflies: you've probably heard this already, but make sure no color value in your scene is higher than 0.8.

Yes and no, the color is multiplied by the texture and by the mix if exist.
LuxRender has a serious problem with high reflective rooms and with mirrors is worst.
An active room has light intensity levels much higher than the intensity of the illuminating light source and LuxRender doesn't know this.
How is this possible? And the conservation of energy? But this happens.
By analogy think about acoustics in a room with big reverberation and echoes and a guitar.

Speaking about skin materials I am using a mix of glossy with translucent and I discovered that if you use translucent or SSS if exist you need a SSS map to modulate the amount.
Some body parts have a significant effect of SSS like the ears and in other parts it doesn't exist;

Stupidity also evolves!