Allowed memory size exhausted

This is a memory issue and can be handled from the wp-config.php file if the server allows.

From what I see you have tried

define('WP_MEMORY_LIMIT', '64M');

which takes care of the front end, but if you still have memory issue in the admin end, then try

define( 'WP_MAX_MEMORY_LIMIT', '64M' );

Please note, this has to be put before wp-settings.php inclusion.

For a more detailed information check the codex here.