Error when trying to add Mastodon username at Vivaldi instance

I would like to cross-post to my Mastodon account but I receive the following error when I try to add my username in settings:

The Mastodon instance returned an error.

Am I doing something wrong here or is this more to do with how Vivaldi have their instance set-up? I experienced a similar problem when trying to add the username to cross-post from a Wordpress install recently.

Can you try entering instead? I think this is an issue when your Mastodon username doesn’t actually match the server hostname.

Thanks @manton, solved. I thought I’d already tried that. So, for users of Vivaldi’s instance, enter