batch - start: "xxx konnte nicht gefunden werden"

start has a odd behaviour. If you have to quote your path of the executable start thinks that this is the given title of the window. The first parameter with quotation marks must be the title of the window. So use "" before the path.

@echo off
REM "": first parameter with quotation marks must be the title of the created window
REM if quote the path to exe because of spaces define first the title of window as e.g. ""
start "" "H:\_Conrad lokal\Downloads\putty.exe" -i H:\ndrhhwikit01_simpel_rsa.ppk -ssh zelckc@ndrhhwikit01.ad.ndr-net.de