As I've stated previously both of my gmail accounts stopped working on 6-Jun. Both of the accounts were using an App Password in order to connect and had worked fine for several months.
I noticed that Vivaldi was displaying, in the status bar, that it could see emails on the server, but none needed to be downloaded.
So I decided to try the following and I now have both gmail accounts working in Vivaldi.
Installed a new standalone version of Vivaldi
Configured one of my Gmail accounts using a new App Password.
This standalone version started downloading the emails immediately in batches of 200.
Once it a had downloaded the emails to about 6-Jun I closed it.
Opened non working Vivaldi and updated the config to use the same App password.
This Vivaldi then started downloading the email as well.
I then repeated the above with my other account and it worked again.
Note: it was only downloading the emails 5-6 at a time, but eventually it has downloaded all of them
Maybe this would work for others as well.