The problem is probably that whatever’s failing is a PHP fatal error that we don’t log. Under normal circumstances for most people those PHP-level errors show up in their Apache logs, but this is happening in a separate process, so it’s not covered by Apache’s log either.
You can try setting PHP’s error_log setting in php.ini (you set it to the path to some writable file that PHP will log to), and trying to run the import again. Some systems have different php.ini files for different PHP environments. If this is the case on your system, make sure you’re making the change in the CLI’s php.ini.
The other alternative way to get at those messages is to manually run the process on the command line, but that involves its own complications (for example, you need to look up the process ID in the database).