Upgrade from D7.22 to D8 - Win7, XAMPP 1.8.1 - bad error-message

[Created on April 9, 2013 at 2:30pm]

I have tested the d7 to d8 upgrade path on my local dev system and first I failed to complete the process.

Win7-64bit german client - XAMPP 1.8.1 (PHP 5.4.7, Apache 2.4.3, MySQL 5.5.27)

XAMPP is installed in path: C:\XAMPP\

My steps:

1. I have made a fresh install of D7.22, profile standard.
2. Then I put the site into maintenance mode.
3. Then I delete all files outside the 'sites' directory.
4. Then I copied all files from Drupal 8.x-dev - Last updated: April 5, 2013 - 14:52 in the root directory.
   Step 4a: Please look at the next comment for step 4a!
5. Then I started http://www.example.com/update.php, which calls http://www.example.com/core/update.php.

[Added on April 17, 2013] The upgrade is successful![/Added on April 17, 2013]

Can someone please write e.g. a page like http://drupal.org/node/570162 ('Upgrade process from Drupal 6 to 7') with the correct steps (first: only main steps) to Upgrade from D7 to D8?
In the file 'UPGRADE.txt' from D8 there are many links to D7-articles on d.o. but not to D8-articles.

Hint:
A fresh install of Drupal 8.x-dev - Last updated: April 5, 2013 - 14:52 was successful.

Wusel