One thing you might check, if you have a Linux (Apache) server, is whether you have a DirectoryIndex entry in the .htaccess file. If you do, and it's not looking for index.htm, that could be a problem. The default search order is index.html, index.htm, and finally index.php. You shouldn't have to do a cPanel/LPCP redirect to get index.htm to run. You might check your .htaccess for other redirects that you forgot about.
If that's not the problem, it could be that your server is misconfigured in some way, but I couldn't begin to guess how.