Allowed memory size of 536870912 bytes exhausted

That query returns every post and needs to load every post in memory. Your site is likely too big for allowing termless searches. You can disable that. If you don’t want to disable it, the result set must be limited somehow.

Thanks Mikko, you’re right I’ve got this on my functions.php file :

remove_filter( 'relevanssi_query_filter', 'relevanssi_limit_filter' );

I will remove this line, and maybe adjust the number of results with

add_filter( 'pre_option_relevanssi_throttle_limit', function( $limit ) { return 250; } );

Solved with the above solution.