Problem: You have enabled the "clean URLs" option that requires URL rewriting, such as Apache's mod_rewrite, but URL rewriting has stopped working and you cannot access the admin interface to turn it off. For example, if you've moved servers and the new server does not have mod_rewrite enabled.
You can disable the "clean URLs" option by adding a new line to the end of your /config.php:
$DP_CONFIG['rewrite_urls'] = false;
After you've done this, you will be able to use the system through the normal /index.php/ URL. For example, you can get to the admin interface via /index.php/admin/.