How to resolve error “Cookies are blocked due to unexpected output.”?

You might be able to narrow it down using output buffering on different hooks… eg.

add_action('plugins_loaded','start_buffer');
function start_buffer() {ob_start();}

add_action('wp_loaded','end_buffer');
function end_buffer(); {ob_end_clean();}

ie. this is a broad test but if the error goes away, the output is happening between those two actions, if not try hooking in earlier (unlikely but could be later too) until it does using the Action Reference as a rough guide.