Found the sollution. Problem was in the definition of the download path. Selenium requires a path with double "\" in stead of "/" separators.

adjusted prefs:

prefs = {
    "download.default_directory": "D:\\Lokale schijf\\COMPANYNAME\\datafiles\\SOFTWEAR_STOCKDATA",  
    "download.prompt_for_download": False,
    "safebrowsing.enabled": True,