What channels were available when you configured the N800 with a German location?

The available channels is most likely driven by the locale. If you configure the N800 with a US locale you probably only get channels 1-12, a European locale may give channels 1-13 and a Japanese locale may give you channels 1-14.

Other than the settings available in the Language & Region control panel applet I'm not sure how else the N800 can determine your region and thus what restrictions to apply to the WiFi channels. That said, you'd think if you chose a more restrictive region (such as switching from Europe to US) while actively using a wireless connection on channel 13 the connection would have to be broken in order to stop using channel 13...

Of course the easy solution may be to change the channel on your access point!