Timezone related fatal error after upgrade to 3.1.2

Hi,

My PHP error logs is full of errors, all same as:

Dec 9 16:33:22 s1 in: PHP Warning: DateTime::setTimezone() expects parameter 1 to be DateTimeZone, null given in /home/.../forum/phpbb/user.php on line 721
Dec 9 16:33:22 s1 in: PHP Warning: array_filter() expects parameter 1 to be array, null given in /home/.../forum/phpbb/datetime.php on line 156
Dec 9 16:33:22 s1 in: PHP Warning: strtr(): The second argument is not an array in /home/.../forum/phpbb/datetime.php on line 120
Dec 9 16:33:22 s1 in: PHP Fatal error: Call to a member function getName() on a non-object in /home/.../forum/includes/functions.php on line 4940

Some members have complained about having random blank pages. I guess this is when this is triggered. Not entirely sure whats going on there, but clearly timezone is not set in some cases.