PDOException: in system_update_files_database() Upgrading from D6.28 to 7.22

I'm trying hard to upgrade my Drupal 6.28 to 7.22 with no success.

I have followed all the steps, but when I try update.php this error happens.

PDOException: in system_update_files_database() (line 2268 of /home/.../modules/system/system.module).
The website encountered an unexpected error. Please try again later.

The function referred is this one, but I don't even know what to do with that. Any idea will be appreciated, I have searched thoroughly but I haven't found much on this error and certainly not a solution.

// Update the record.
if (count($updated_fields)) {
db_update('system')
->fields($updated_fields)
->condition('filename', $old_filename)
->execute();
}