Change the license:url value given for the webfonts

The seven.libraries.yml file contains the following lines.

webfonts:
  license:
    name: SIL Open Font License, Version 1.1
    url: https://openfontlicense.org/
    gpl-compatible: true
  css:
    theme:
      css/theme/fonts.css: {}

The url value is wrong: It should be the URL for the license, in this case https://openfontlicense.org/documents/OFL.txt (or https://openfontlicense.org/open-font-license-official-text/).