iPod touch 5.1 Wi-Fi Software Update doesn't work

Hi.

My existing iPod touch (firmware 5.0.1) Wi-Fi connection to my router works fine.

However I tried to update to firmware 5.1 now. No success. The Software Update starts showing 174 MB to go, but the progress bar doesn't fill. After #5 minutes, the iPod touch shows a failed upgrade message.

When I connect this iPod touch to an iPhone 4 Personal Hotspot (which is connected via 3G to the Internet), the update starts!

Are there any known issues doing a Software Update using router?

I use a FRITZ!Bos 7390.

Thanks,