The error seems due to the file /wp-includes/functions.php on line 1368 not on a newsletter plugin file. Where did you see that error? Opening which panel?
While subscribing from the site to the newsletter? Can you give me the page to make a test? Anyway, Newsletter does not use that wordpress function on frontend, so this is very strange… which version of wordpress are you running?
Since the error is raised from a wordpress file, it seems the wordpress has not completely update…