Push down the first line in a grid without indention?

Questions
2

Thank you for posting the code with the correct formatting. However, the code as you presented it is not a minimum working example (MWE). When I paste it into a blank document it fails to find your logo or the fonts you specify. Also, is this your actual contact information? You may want to remove it.
Please read How to post in the Questions category for more information.

As for the problem, you can use v(18pt):

#grid(
  columns: (auto, 0.35cm, 1fr),
  rect(width: 4.1cm)[logo],
  [],
  [
    // #box(height: 18pt)
    #v(18pt)
    #text(weight: "bold", size: 17pt)[IT-Services & Consulting My Name] \
    #par(leading: 6pt)[
      #text(weight: "light")[
        Street and number, 1234 Musterort - Deutschland \
        E-Mail: info\@unknown.de \
        Tel.: +49 (0)123 1234567 | Fax.: +49 (0)123 1234567
      ]
    ]
  ]
)

Note that I also replaced your logo with a rectangle, and removed the fonts entirely.