Today we had a similar problem on SLES 11 SP4 with Apache 2.2 with an pretty much default apache configuration. Problem was, Apache ignored an existing index.html file when not explicitly called on the url. e.g. server.domain.tld/dir/
However if called with explicit file, it serves the content. e.g. server.domain.tld/dir/index.hmtl . so there was no problem with filesystem permissions. All Files under htdocs had sufficient permissions for Group OTHERS . BUT all the htdocs files back then where owned by root.root . So in this "Mode" Apache did not serve index.html if not mentioned in the URL.
The Moment we recursively changed back the htdocs directory and files back to owner "wwwrun.www" which is "default" on SLES (suse linux enterprise Server) without changing the permissions itself, apache started to serve the index.html again, as usually expected. No restart nor nothing needed.
cant tell if this is a bug or a feature. just wanted to let you know what we have experienced and what solved the problem.
so you might check on your ownership of your htdocs files and make sure its owned by the webservers service-account and group.