When attempting to delete a user who has undelivered PMs, a SQL error occurs because a user ID is not provided to the query.
SQL ERROR [ mysqli ]
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 4 
UPDATE phpbb_users SET user_new_privmsg = user_new_privmsg - 1, user_unread_privmsg = user_unread_privmsg - 1 WHERE user_id =
FILE: (not given by php)
LINE: (not given by php)
This appears to be caused by PHPBB3-10605.