json error causing WP slowness

I am running a plugin called Query Monitor on my WP Multisite (using WPMUDEV Multi DB) and see the following php error on every page load no matter whether I'm on the default wp 15 theme or in any admin page.

Warning json_encode(): Invalid UTF-8 sequence in argument 6 wp-includes/functions.php:2663

I tried deactivating all of the plugins to cut out any variables. I'm running WP 4.1. Any ideas on what the issue could be?