That sounds as though you are added them within DokuWiki. HTML files are forbidden within Doku itself. The docuroot I was referring to is the webserver's root, often Apache on a *nix box. If you are running Linux, have a look in /etc/httpd/conf and search httpd.conf for it:
RAW # grep -i documentroot httpd.conf
# DocumentRoot: The directory out of which you will serve your
DocumentRoot "/var/www/html"
# This should be changed to whatever you set DocumentRoot to.
# DocumentRoot /www/docs/dummy-host.example.com
In this instance you would need to put the files under
/var/www/html[/m]. Using your original file it would need to be [m]/var/www/html/a/b/c/index.html[/m]. The URL would then be [m]http://<hostname>/a/b/c/index.html
There is a rationale to this. The nature of a wiki is to allow arbitrary users to edit files (pages) within the wiki. A maliciously edited DokuWiki page is annoying vandalism. A maliciously edited HTML page could run arbitrary code and do a lot of damage. Have a read of
https://www.dokuwiki.org/mime