Hallo Paul,
- I am not allowed to turn on "Google Fonts". Otherwise the fonts will be loaded via Google servers.

What does "installed fonts (2)" mean here ? are they not "local" after "installation" ? Why do I have to "install" if these fonts are not installed locally? I can use google fonts in my pages without installation. Totally misleading...
- I go to "font.google.com" and download font-set. Then I go to "Font Squirrel Webfont Generator" page and upload TTF file. After that I get an error message every time, although the TTF file is not faulty.

Apparently the page can not handle all TTF files!
I have provided my TTF file as a ZIP. Can you test if you can convert or upload?
Why in "Font book" not allowed to install simple woff or woff2 files? must I necessarily use ZIP file from fontsquirrel?
P.S.: alles clar -> alles klar ;-)