Let’s check if this has something to do with the browser,...
Let’s check if this has something to do with the browser, ewanbcn.
You can use an incognito browser or a new private window. The web browser keeps files in its cache and saves the pages you have previously visited. Sometimes these files can affect communication between the browser and QuickBooks Online and can cause issues like this.
If you’re able to sign up through this gateway, try clearing the cache of the browser you're using or use a different one.
Though, if the same thing continues, it would be best to contact the support team. They’ll be able to help you investigate and fix the issue.