Okay, I've solved the issue.
First, I updated the Kindle to 4.0.1. Then I used the '.assets' trick to remove the ads: http://blog.the-ebook-reader.com/201...-paying-extra/
Then I rebooted into diagnostics mode, calculated the root pw (with this page: http://members.ping.de/~sven/kindle.html
). Then I did the following:
mount /dev/mmcblk0p1 /mnt/base-mmc
mv 600x800 600x800.bak
ln -sfn /mnt/us/screensaver /mnt/base-mmc/opt/amazon/screen_saver/600x800
Then I copied all the default screensavers from 600x800.bak to the /mnt/us/screensaver directory. After reboot, it all worked.
Note: Make sure the screensavers you put into the 'screensaver' folder are color RGB pngs and *not* greyscale ones.