Unfortunately, it looks as though the server has a poorly configured CGI link to PHP. If you can change this (i.e. if you're administering your own server), I'd suggest using PHP as a module or as CGI using the mod_actions module; either of these configurations support the required PATH_INFO variable.
If you can't administer the server or don't want to change the CGI configuration, you have two options: either host the script somewhere else, or wait until the end of January. We'll be releasing OJS 2.1 with a configuration option that avoids the PATH_INFO problem, although the URLs will be uglier.
Open Journal Systems Team