Had similar problems, tried every method suggested and resolved it when I noticed some extra code in the main web folder .htaccess file which I deleted and now everything works fine. Code below and what was deleted.
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
………………………………….
the following text, which is not present on other websites, has been deleted, not sure why it is there.
AddHandler application/x-httpd-php .ico
AddHandler application/x-httpd-php .png
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{QUERY_STRING} ^[^=]*$
RewriteCond %{QUERY_STRING} %2d|\- [NC]
RewriteRule .? – [F,L]
</IfModule>