Y-Axis offset or backlash

Dear all,

I have already put a post about the following issue into the German forum ( https://www.stepcraft-systems.com/forum/fraesen/6439-y-achsenversatz), but I figured I might have a better chance of getting response by asking here, too, as the audience seems to be a bit larger.

In brief, I'm observing an offset of about 1 mm for the y-axis. The drill is not at the exact same position at the end of a run as compared to its beginning.

Some basic information:

Model: Stepcraft 2-300
Spindle: HF-500
Material: 3 mm plywood
Feed rate: 4 mm/s
Spindle speed: 20000 U/min
End mill: 2-Flute fish (downcut) 1 mm

I have already removed old grease from the lead screws and linear guides, cleaned them and replaced it with new grease. All axes appear to move smoothly at slow and rapid speed all the way.

To illustrate the issue I have, please see the following test I've made:

The innermost square is 50x50 mm. The distances between the paths closest to each other should be 0.5 mm throughout. After doing a bit of research, I'm wondering if it might be a backlash due to too much play between the lead screws and nuts. From the test you can clearly see that the milling direction (i.e. clockwise or counter-clockwise) affects the result.

Any advice that might help me in resolving this issue is very much appreciated!