The parquet has black stains from cleaning water - What to do?

I use the "Woca Fleckenentferner-Spray" or "Wood Stain Remover". So, it’s the same, sometimes it’s only labeled in English, sometimes in German. The stuff costs about 15-20 euros in a small spray bottle. But it really works well. Otherwise, you can also look for "Hausmittel Oxidationsflecken Holz". There is something for that too. It didn’t help me very much back then. I don’t even remember exactly what it was. Good luck!