The problem with the & laquo; & raquo; in the html <title> tag (page title)

These html entities are converted to & amp;laquo; and & amp;raquo;

How can I fix it?

It's the theme that most probably escapes it. Example: https://github.com/getgrav/grav-theme-antimatter/blob/develop/templates/partials/base.html.twig#L8 ( see |e).

You can remove the escaping, or use the already unencoded character («) in the page title.