Can't update to Drupal 7

Hello,

I'm trying to update a local Drupal Installation (latest 6) to Drupal 7.22.
After starting the 'pending' updates, there are some db-tasks written into the settings.php and than the update process is aborted by an AJAX HTTP error.

I found different solutions for this- my memory_limit ist set to 256M now in the /etc/php/apache2/php.ini

the base_url is set to http://localhost:8080, the
$db_url = 'mysql://root:xxx@localhost/drupal';
which both worked fine with Drupal 6

in the conf.d folder of my local php installation there are the both files for PDO
pdo.ini
pdo_mysql.ini

Besides some warnings like
Notice: Undefined property: stdClass::$disabled in _node_types_build() (line 747 of /var/www/modules/node/node.module).
the latest error is
PDOException: SQLSTATE[42S02]: Base table or view not found: 1146 Table 'drupal.block_custom' doesn't exist: SELECT bid, info FROM {block_custom} ORDER BY info; Array ( ) in block_block_info() (line 208 of /var/www/modules/block/block.module).

What else could I try to change on my system to get this upgrade?

Thanks a lot for any help
regards

Adriana