Sorry for this very late response! We were pretty busy for the last weeks…
Yesterday we found the solution for the problem. The left sided belt slipped through at some time, in follow the belt had different tensions upside and downside and it follow of this the y-axis moved slightly diagonally. We moved the left side mirror first by hand a bit and saw that this improve the result. At last we slightly loosen the gear of the backside step motor and with this we distribute the tension even. After this the problem was solved. 
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