3.7 update Fatal Error

I’m attempting to up update to WP 3.7, but I keep getting error messages. I tried switching back to the Twenty Twelve theme and read other related threads and contacted my host (1&1), and we troubleshooted permissions issues, but that did not fix the problem.

The 1&1 guy finally said he was maxed out and suggested taking up with WP.

This is where I’m at now:

Fatal error: Call to undefined function mbstring_binary_safe_encoding() in /homepages/20/d387371247/htdocs/clickandbuilds/WordPress/Slucherville/wp-admin/includes/class-wp-filesystem-direct.php on line 62

Any ideas on how to get past this?

Thanks