I was in the same situation. I am a new user to Quickbooks Online, and this was the first time I had tried to import the data from my bank. The only thing I found which worked was to right click on the HTTPS padlock icon to the left of the URL bar for the Natwest web site, then click "site settings", then under Usage, click "Clear data". After having tried most of the other recommendations in this thread, (except trying removing only the cookies for Natwest's online banking only, also via right clicking the padlock icon, rather then clearing all browsing data), I still had had no luck getting Quickbooks Online to connect to Natwest. Quickbooks (which was open in an incognito Chrome session at the time, with Natwest's banking in a normal Chrome ) successfully uploaded the Natwest data as soon as I had cleared the data for Natwest's online banking, as described above.
