@Booster First of all, I don’t have this issue, so it’s only a suggestions, not some fix that I did try myself.
…this being said…
Be gentle - take care not to force anything. Screws shouldn’t be too tight - when tightening a screw, hold the allen key by the short side. If you use the long side, you’ll have too much “power”…
Standing in front of the Falcon 2:
A. From right to left, just behind the front of the machine, there’s a rod with 2 cogwheels at each end through which the belts pass on each side - it’s this rod, with the motor in the middle, that manages the laser’s back/forth movement.
B. On each side of the device, you’ll see an oval hole with a screw in it - this is the screw to secure the tensioner and prevent it from moving once adjusted.
C. At the rear of the unit, there are 3 screws (vertically) on each side. The top 2 are for securing the frame - the bottom one is for the tensioner.
First of all, I’d check if the frame itself is square. Use a carpenter square or something similar that you are sure is square and check all corners. Or you could have the laser cut a piece of thin wood like the one that came with the device - that should create a proper square piece even if not parallel to the front 
Alternatively, you could mesure the distance between the front-left + back-right corners, and do the same with the front-right + back-left corners - your 2 measurement should be identical!
If the frame isn’t square:
- Release the 2 frame screws on each side on the back (See C above)
- Try to gently move the frame to be square and tighten back one screws from each side, but don’t tighten too much yet.
- Play with the other screws and move the frame if necessary to try to get as square as possible.
- Once satisfied that your frame is really square, tighten the screws trying to keep the frame as is - try not to screw all this hard work
- Once all is screws are tightened up properly, check the frame for squareness - repeat steps above if necessary…
If/when the frame is square and you still face the non-paralelism of the arm and front of the unit, here’s what I’d suggest :
- Choose one of the sides to work on - you shouldn’t need to do it on both sides.
- Loosen the screw securing the tensioner (see B above)
- Using the bottom screw at the rear of the unit (see C above), release the tensioner enough to allow you to “jump” one or more teeth on the gear (see A above) and move the laser arm parallel to the front of the unit.
- Once you’re happy with the alignment, apply tension with the bottom screw at the back - try to give the belt the same tension as the belt on the other side of the unit.
- Re-tighten the side screw (B) to secure the tensioner and prevent it from moving.
Let us know if it works.