If you have a CFW on your Wii U it is recommended that you block updates to it by changing the following: (Taken from Wiiu.guide)
The listed DNS servers will block various Nintendo URLs that the Wii U communicates with. In other words, these servers are built to prevent updates to your Wii U. You need to set two of them, because if one goes down – the other will kick in and work.
Step 1) Enter Sytem Settings from the system menu.
Step 2) Select ‘Internet’ for internet settings.
Step 3) Select ‘Connect to the Internet’
Step 4) Connect to the internet. If you have already configured an internet connection, Press ‘X‘ to display connections.
Step 5) Select your default connection
Step 6) Select ‘Change Settings’
Step 7) Find the DNS section and select it.
Step 8) Select ‘Don’t Auto-obtain’.
Step 9) Set both of the following to the ‘Primary DNS’ and ‘Secondary DNS’.
Step 10) Hit ‘Confirm’
Step 11) Press ‘B‘ to save