I thought, well, can happen even once and contacted the customer service. After I had described the defect and some photos posted, I received the following reply:
"Dear xxx,
Thank you for your letter.
We regret to inform you that we can not consider your complaint because the apparent damage in the photos indicate improper use.
Unfortunately we can not offer free replacement due to its.
We therefore ask for your understanding that we can get closer to your claim for this reason alone not.
However, we can offer you as existing customers who need spare parts cheaper (15 euro). "
TL, DR:
The quality of the chair is undercut only by the quality of customer service. Never again!