[NiLuJe]

@f-j-p: The first error is a corrupt file. Re-download the update & re-upload it again, and take care to unplug your Kindle safely.

As for the jailbreak, I can't tell you any more without more details about the error you encountered. (But the current version should be listed in linkjail/info.txt, and the upgrade instructions are one of the first things in the first post of this thread).

@Mickra89: Unrelated. Neither of those hack do anything even remotely affecting that part of the system. (I'll add that, depending on the current load on your device, there might be a somewhat noticeable delay before the WiFi comes up. If you have a large library/archive/collections, let your Kindle sit still for a while. Also check that your kindle is not sitting looping trying to index a broken file). You might also try again after a full reboot of your device, who knows ;D.