The customer service people quickly answered my question. For future reference:
There was a hidden file, named .htaccess, which you couldn't see. I've
deleted that file, go ahead and attempt the install again and you
A hidden file is any file whose name starts with a dot.
If you're using FTP, there should be a checkbox in the client settings
whether to display hidden files.
If you're using SSH or telnet, you can add the -a flag to the 'ls'
command to see hidden files: ls -a