If you an error that looks like one of these...
Warning: require_once(/home/username123/public_html/amember/zend.inc) [function.require-once]: failed to open stream: No such file or directory in /home/username123/public_html/amember/rconfig.php on line 19
or
Fatal error: require_once() [function.require]: Failed opening required '/home/username123/public_html/amember/zend.inc' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/username123/public_html/amember/rconfig.php on line 19
This error is saying that the file in /public_html/amember/zend.inc doesn't exist, or isn't readable. Check and see if this file exists. If it does, check the permissions on this file to make sure they are 644 or 755.
The file /public_html/amember/rconfig.php is simply the PHP file that tried to call "zend.inc". The PHP script must call the file by its exact name, or you will get this same error.