Alright,

I'm moving my first steps with Mantra.
I did a studio setup, which looks something like this:



I'm tweaking the render settings in order to understand how to speed up Mantra and keep acceptable render quality.

That being said, no matter what I do, this shadow issue keeps showing up:



I'll keep investigating though right now I'm clueless.
If anyone has an idea why this is happening…

Try scaling up your Area lights. They are probably at the default size of 1x1. I usually start at 4x4. You might also want to switch over to Physically based attenuation for each light as well. Change the Mantra node to Physically based too!

Thank you Enivob,

yep, I already have physically for both, lights and mantra.
I'll try tweaking the lights size.

I'm a beginner in houdini, but maybe try to increasing th sampling in the render setting. Maybe you alredy did it ^^

Enivob got it right,

it's the size of the lights.
Increasing the size fixes the shadow issue, of course now it's a matter of tweaking intensity/position/rotation in order to keep the same kind of result.

Edit: although, now that I'm thinking again about it, as a 3ds Max user I used this trick to get the type of shadow I needed. In other words, if I want a kind of harsh shadow, but still using area lights, I simply make the lights smaller and I increase the intensity. In fact right now my shadows are softer, which makes perfectly sense.
There must be a way around in Houdini then, in order to get the same of type of shadow yet avoiding the edge overlay issue.
Maybe using another light for shadows only, which probably is the Phantom option, but I can't find any render tab options for the selected light.