@Ange27007 My issue started 2 months ago as well and I have Netlfix from Sky for 3 years as well. Chodley also mentioned that people are having this issue for 6 months or more which suggests that this is definitely an issue with the auth token for the Netlfix app. It looks like the Netflix auth token was set/configured to expire after 3 years. Every customer who has logged into Netflix continously for 3 years is going through this issue. Netflix should have informed its customers by emailing them before hand suggesting that customers log off from the Netflix app atleast once a year to renew their auth tokens annually. In your case, it looks like even though you have signed out of the SkyQ box remotely through a web browser, there is some remnants of old auth token cached on your SkyQ box that is conflicting with the new token whilst you can log in sometimes and fail at other times. There are 2 options for anyone having this issue. I will give the instructions below but make sure you reboot/restart your SkyQ box after the steps.
If you cannot log into Netflix through SkyQ box:
1 - Log into Netflix via a web browser.
2 - Highlight your profile picture (Top right corner).
3 - Click on Account.
4 - Under the Security and Privacy option, click on Manage access and devices.
5 - Select the device name: (Sky UK Set-top box).
6 - Click on the "Sign out" option on the Sky UK Set-top box.
7 - Go to your SkyQ box and perform a complete power cycle on the SkyQ box (not just on standby mode)
8 - Launch the Netflix app on your SkyQ box.
If you manage to log into Netflix on the SkyQ box:
1 - While you are in your Netflix profile on the SkyQ box, scroll down and on the right hand corner and click on/select the 'Get Help' option.
2 - Sign out of Netflix.
3 - Sign back into Netflix (with your email address and password for your Sky Subcription if you don't have a separate subscription for Netflix). Alternatively, sign in with your Netflix credentials if you have a separate Netflix subscription.
4 - Power cycle (on and off) your SkyQ box. Make sure the power is completely out (not just on standy mode)
Hope the above helps anyone with the same issue.