An SQL error occurs when adding a new address as the primary address to the address book. I looked into this and the error occurs in includes/modules/pages/address_book_process/header_php.php around line 222.
Replacing the line
$sql_data_array = array('fieldName'=>'customers_firstname', 'value'=>$firstname, 'type'=>'string');
solves the problem as far as I can tell.
$sql_data_array = array( array('fieldName'=>'customers_firstname', 'value'=>$firstname, 'type'=>'string') );
Initially, $sql_data_array is created to store data for a query that is performed on the address_book table. Once that query is executed, $sql_data_array is reused to execute another query, but this time on the customers table. The problem is that the variable is not reset and still holds data which corresponds to columns in the address_book table - columns which don't match up to the customers table. :)