What version of WP User Manager and WP User Manager Username Length are you running?
What is the fatal error? If you can’t see the error message, can you try editing your wp-config.php file to replace this line –
define( ‘WP_DEBUG’, false );
With these lines –
define(‘WP_DEBUG’, true);
define(‘WP_DEBUG_LOG’, true);
define(‘WP_DEBUG_DISPLAY’, false);
This means that any errors are logged to `/wp-content/debug.log’, because some errors are not visible on screen.
More details about that at http://codex.ww.wp.xz.cn/Editing_wp-config.php#Debug
Can you then retry the process and send me the debug.log file that should be created.