I am having this same issue (404 File Not Found) on several pages (not just WATCH, but also when you click on your login name to go to your user page, etc).

The path to files are correct (triple checked). Chmod all files that need chmod (triple checked). All files uploaded (triple checked).

The only thing is that I'm using the default URL of the host provider that the script is being hosted on and not a domain name (yet). I have the domain, it's just that I don't want to set the nameserver for it to point to this host until I have it working and customized. This host supports YouTube Clones so it's not a .htaccess issue or anything like that.

Any other ideas or will this script simply not work without a "regular" domain name?

ANSWERED MY OWN QUESTION: This script will not work without a "regular" domain.

I went ahead and set the nameservers on the domain, fixed the config.php to reflect the url and viola! Everything worked just fine.