Font looks different & fontspring bulletproof syntax

Hi,
I followed your instruction “how to add custom fonts”.
Unfortunately I get in the style.css the message: “font-face declaration doesn’t follow the fontspring bulletproof syntax”. Google mainly finds solutions directing to a question mark topic – if I add following line this error disappears:
url('orkney-medium.eot?#iefix') format('embedded-opentype'),
but your instruction implies not to have this line.

However the main issue persists: The new font (orkney-mediuem as woff) appears in the customizer/Typographie section as supposed but looks quite different than should be (serifs applied). I tried different sources of the font with no improvement. Some sources provide a .woff as well as a .woff2 file, both showing this issue.