Hi.
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"Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 20480 bytes) .../.../...php on line ......"
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268435456 bytes equal to 256Mb. This is greater than 128M.
You can simply change memory_limit from
define('WP_MEMORY_LIMIT', '128M');
to
define('WP_MEMORY_LIMIT', '512M');
also you must define memory_limit for admin wordpress user
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
If this will be resolve your issue - you can pay me few dollars tip.