Works now, maybe I did something wrong.
I now inserted
DefaultFormatSettings.ThousandSeparator := '.';
DefaultFormatSettings.DecimalSeparator := ',';
in the formshow-event of reportdesign and when I use
[formatfloat('#,##0.00 €',strtofloat(d.Preis))]
I get 10,10 € what is right!
With [formatfloat('#.##0,00 €',strtofloat(d.Preis))]
I get 10.10000 €
I think I interpreted the format string false
It also works from my project.
So the format issue is solved for now!