I’ll try my best to guide you trought some ideas that could solve your problem.

1 – Make sure your .htaccess is set to default

If you check the WordPress CODEX, you could compare your default .htaccess to the one you have on your root folder.

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Source : https://codex.wordpress.org/htaccess

2 – Create a new homepage

You could try to create a new Page and go on Settings > Reading then at the option Front Page Display try to check the option « A static page (select below) » and get your new created page.

After you could try to update the Permalinks while setting them to Default.

3 – Try to install a freshly new WordPress

The last solution for me would be to create a completly new WordPress and check if the problem still happend without additionnal modifications.

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If you could send us a link or some errors, it would help us to help you !