Ok, the problem is caused by even screen sizes (not really that, but it is the easiest way to describe the problem) - then CSS that counts elements width cant divide width correctly, and so 1px spacing can be seen, and under that spacing - the background that can be seen when grid items are loading can be seen.
Right now the problem can't be solved for everyone, as every user's grid elements differ, have different grid elements's background colors.
We are researching for a better way of placing grid elements, but right now there is no 100% working solution.