this works for both situations:
/subfolder/0001/index.html and
/subfolder/0001/
Code:
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} ^(/.*)/index.html$ [OR]
RewriteCond %{REQUEST_URI} ^(/.*)?/$
RewriteCond %{REQUEST_URI} !^/index.html$
RewriteRule . /index.html [L]