After 3.8.1 upgrade, table_prefix problem (I think)

I have WP hosted on a Parallels virtual machine on a Blacknight server. I have used AIO to change the database table prefix. Last night I used Blacknight’s control panel to upgrade WP, and it changed the wp-config.php file.

In the database there are two sets of tables, one with the wp_prefix and the other with the xxx_ prefix (names changed to protect the innocent!) With the table_prefix in wp-config set to wp_ the sample Hello world page comes up. If I change the table_prefix entry to xxx_ I get a blank web page. With table_prefix set to wp_ I changed the posts table in the database so the post name was Bonjour rather than Hello world, and that change appears on the web page.

So WP can successfully access the database tables as wp_, but not as xxx_. What am I missing? And how do I fix it?

https://wordpress.org/plugins/all-in-one-wp-security-and-firewall/