I started getting this error back in version 3.7.X and despite a couple of upgrades, it won't go away.
Oddly, it only occurs with two of our 15 blogs, all running on the same machine, all with the same version of WP
[Wed Jun 04 08:17:15 2014] [error] [client 10.1.7.30] PHP Warning: json_encode(): Invalid UTF-8 sequence in argument in /srv/www/ht
docs/path/to/blog/wp-includes/update.php on line 270
Anyone got any ideas? TIA!