RewriteCond "%{THE_REQUEST}" "\s(/[\w/-]+?)/index\.html[?\s]"
RewriteRule "^" "%1" [R=301,L,C]
RewriteCond "%{DOCUMENT_ROOT}/$1/index.html" -f
RewriteRule "^([\w/-]+[^/])$" "/$1/index.html" [L,T=application/x-httpd-html]
Observe o DirectorySlash
possível problema.