httpd error log had the message PHP Fatal error: Call to undefined function json_encode() in /var/www/html/DrasticTools/drasticSrcMySQL.class.php on line 178, referer: ...
The reason is that Centos ships with PHP 5.1, which does not include JSON support.
Updating PHP on Centos fixed that problem.
I used the following instructions to update PHP on Centos:
wiki.centos.org/HowTos/PHP_5.1_To_5 ...
Note that when it says 'Copy/paste the following into this file:', you should delete any leading blanks on each line; otherwise you will get error messages from "yum update".