From b2evolution manual

Enable Debuglog

To have a Debuglog with a load of information on the bottom of each page, set

$debug = 1;

in /conf/_advanced.php.

Blank pages

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.