When the machine cuts along the X axis it burns while along the Y axis the cuts are perfect. I tried turning the panel 90° but the problem remains.
I can’t run the tools on the material with lightburn because when the laser engraves the squares one side is burned while the other (90°) is perfect.
I also tried with air assist but the problem doesn’t resolve itself.
What can be the cause? 
Is this an LED or CO2 laser?
It is a led
LED laser beams are fairly elongated ovals in cross section.
There are some (more expensive!) laser heads with special aspheric optics to reduce this asymmetry.
there are replacement lens which are called “compound” IIRC and they do a better job at refining the elongated LED lasing die to a point. I think I paid maybe $15 on ebay for one for my Ortur 15W(consumed) LaserMaster1 and will see if I can find the link and post it. Sometimes the lens’s are called G2, G7, G8 and my tests showed the 3 element G7 gave me the best results for a more consistent X,Y dot shape.
$10 - 3 Element Glass Collimating AR Lens 405nm - 445nm - 450nm Laser Diode M9/P0.5
Somewhere I also posted a link to a video which digs into the LED die shape and will try to find that too.