I started this thread some time back. After much troubleshooting I found a solution to my issue.
I had deleted the Now apps and undertaken a factory reset of all the Roku devices numerous times I was having no success and Now support were not really understanding the problem. I realised that when undertaking a factory reset Roku asks you to sign into your Roku account, this was the one constant I had not tested.
Using a Roku Express I did a factory reset but this time generated a new Roku account. Loaded the Now app and could access My Account and log in. I did another reset using the old Roku account and fault returned. Factory reset all my other Roku devices (4K stick and Streambar) and using the new Roku account everything working.
I am not sure why the Roku account should stop you logging into Now but it appears it does.
may be worth you trying a new Roku account on one of your devices to see what happens.