After a server upgrade I am suddenly getting a blank page with this error on all K2 pages

Fatal error: Call to undefined function mb_regex_encoding() in /home/...../..../...../components/com_k2/helpers/route.php on line 94

Joomla! 3.4.8
K2 v2.6.9
PHP 5.5.33
MySQL 5.6.29
Apache/2.4.18

I know I can comment out the calls for mb_regex_encoding in k2category.php , k2item.php and route.php and the problem will go away, but I would like to know what is going on.

The multibytes PHP module is missing from your server.
Install it and K2 will work as intended.