All pages, articles and services give the error 404

Hi All,
I have a big problem with Sydney: all pages I create (pages, articles, services etc.) are not found, both when I clic on “preview” or when I clic on the links from the homepage. The only page that works is the homepage.
I already tried all the fixes which have been suggested in this support forum (modifying .htaccess, changing permalink settings, flushing permalinks, adding custom css). Nothing worked.
If I move to a different theme, I can see the pages, so the issue lies on Sydney. What can I do?

Hello there,

Have you tried deleting your existing Sydney theme through the Theme administration panel (Appearance > Themes) and have installed the fresh one?