@Basshunter - here are my recommendations:
option 1) Use 3ds Max native lights instead of Corona lights (if possible). Corona lights are more realistic, they always have some area, they have directionality, etc, but that makes them harder to render. 3ds Max lights are "fake" but they will render faster. I am not sure if it is possible to make 3ds Max lights have a visible disk shape rather than always being shot from a point, which may not work for you...
option 2) Disable Adaptive light solver in the Performance tab - looks like currently there is some issue here and this should be improved. The scene might render a bit slower but you will immediately get rid of the square artifacts.
option 3) Leave the Adaptive light solver enabled, enable the Development/experimental stuff rollout (this:
https://coronarenderer.freshdesk.com/support/solutions/articles/12000021288 ), scroll down to "Lights" and change "Solver" from the default "Select automatically" to either "Better object-based" or "Scalable". Both seem to be working fine and without the square artifacts.
I would also advise increasing the Light Samples Multiplier to a higher value than the default 2 (probably ~4 will do the trick).
Attaching some example images and will send you scenes in PM.