Normally I would accept this as a solution but not yet.
So I already checked that I was on latest version of Roku Firmware, I had updated it about a week to 10 days before I made OP.
I also tried deleting YouTube App, doing hard reboot and re-installing, this did not work.
In frustration I tried the Settings/System/System update/Check now each day only to be told it was up to date and still the app did not work up to 10th June, then later on 10th repeating Settings/System/System update/Check now, did allow it to work BUT on 11th the problem came back. So I did Settings/System/System update/Check now again several times through the day on the 11th and later on in the day it worked.
I have have no idea what is going on "under the hood" but I have read of a dispute between YouTube and Roku, different reports online as to whether resolved or not. So I am going to do more testing over the next few days.
I think that there may be a certificate issue and that certificate may have a daily expiry date set rather than than annual, if things are being negotiated I can see this as a bullying tactic.
I also noticed the UI on YouTube app is worse, only showing 3 instead of 5 videos across and only 1.5 rows down at a time.
It also lost all my searches and recommendations history (I never use Google Accounts).
I still blame Google for these problems rather than Roku but Google is a brick wall and we are clients of Roku so it is right to post here, especially as this may be evidence in decisions t break up Google and YouTube.