Drupal 8 RuntimeException: Failed to start the session because headers have already been sent by “core\lib\Drupal\Core\Render\Renderer.php”

 

Restart the server and empty the cache tables, you can run following SQL, then find your error and solve it.

TRUNCATE `cachetags`;
TRUNCATE `cache_bootstrap`;
TRUNCATE `cache_config`;
TRUNCATE `cache_container`;
TRUNCATE `cache_data`;
TRUNCATE `cache_default`;
TRUNCATE `cache_discovery`;
TRUNCATE `cache_dynamic_page_cache`;
TRUNCATE `cache_entity`;
TRUNCATE `cache_mailchimp`;
TRUNCATE `cache_menu`;
TRUNCATE `cache_page`;
TRUNCATE `cache_render`;
TRUNCATE `cache_toolbar`;

 

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