Reply from Sport-Thieme team:
Many thanks for the pictures, that you have provided simultaneously. Since the aluminum underneath also has visible marks, it seems that there was a harder impact on the grips, as you can normally expect on regular use. So, we would not go with a warranty case due to general minor quality of the stilts.
However, we will send 4 new pairs of foam handles. Please check as a long-term solution, if you should rather "pimp" your stilts (due to the intense use) with stronger grip handles of rubber. A local seller for bicycles surely has some suitables.
As short term solution, we can deliver 4 new pairs of foam grips.