I found two errors in my LocalSettings.php:
1. I tried to load an extension, whose files I hadn't downloaded. This was the reason for the updater to stop at the second page.
2. the following line was commented out: $wgCacheDirectory = "$IP/cache"; for an explanation, see: http://serverfault.com/questions/743898/mediawiki-not-displaying-style-inter...
Best wishes, Andreas