Ok, first this, how are you using Netflix on the WiiU, Netflix was cut off like over a year ago. Is there a way?
Second, I think I just discovered something else 5.5.5 changed.... maybe?
I had my console on 5.5.3 for like a year and a half or so after 5.5.5 came out. I had it DNS blocked and it showed the download for the update as failed in the Download Management thing. However, I was able to use the Youtube app and log on to the Nintendo Network just fine. Never had a problem with this failed update download in waiting, wasn't forced to download it.
However, this new 5.5.6 update came along (well after I updated to 5.5.5), it showed in Download Management as failed download, like normal, but my console suddenly would not allow me to do anything involving being logged in to Nintendo Network, it logged me out and wouldn't even let me open the Youtube app because it wouldn't log into the Nintendo Network. Instead, it said something like "you must update to log into the Nintendo Network". Forcing you to have to download and install the update, regardless if the update was DNS blocked.
As soon as I turned the console on, it had me logged out of Nintendo Network so it immediately popped up this message while trying to log into it as well.