From b2evolution manual
To have a Debuglog with a load of information on the bottom of each page, set
$debug = 1;
Blank pages often mean that there's a fatal error during the PHP script execution and the website/server is configured to not display them.
You can try to look into the server's error log or add the following code to /conf/_basic_config.php (/conf/_config.php before b2evo 1.8):
ini_set( 'error_reporting', E_ALL ); /* report all errors */ ini_set( 'display_errors', 'on' ); /* display them on the page */
Add this after the opening "<?php" tag at the beginning of the page.
Note that this won't help, if you have a parse error in the config file you're editing itself, because PHP cannot execute the statements that change the reporting..