Hi, I’m trying to log in to social.vivaldi.net via Brutaldon, because it currently looks like Mastodon v5.0 won’t work on my maxxed-out iOS15.8.8 phone. I’ve tried on 3 devices, only one of them successfully. I don’t know if this is a Brutaldon problem, or a Vivaldi one, or a device one, so I’m asking here. (I searched, but found no topics about Brutaldon).
Here is what happens:

A )
Mac OS 14.7.4, Brutaldon v2.15.0
I go to https://brutaldon.org and click Log in , which takes me to
https://brutaldon.org/login
I enter…
https://vivaldi.net
…and click Log in > Server Error (500)
Trying again with
https://social.vivaldi.net
… goes to
https://social.vivaldi.net/oauth/authorize?client_id=G..[long string of characters]...j_E&response_type=code&redirect_uri=https%3A%2F%2Fbrutaldon.org%2Foauth_callback&scope=read+write+follow&force_login=False
Gives me a Vivaldi authorise page, Click Authorise and I’m in.
Great! But I don’t actually need to use Brutaldon on this machine, logging in to the instance directly in Vivaldi browser works fine.

B )
iPhone7 iOS15.8.8(max), Brutaldon v2.15.0
Firefox 125.3 (41348) or Safari 15.6
I go to https://brutaldon.org and click Log in , which takes me to
https://brutaldon.org/login
I enter…
https://social.vivaldi.net
…and click Log in >
https://social.vivaldi.net/auth/sign_in
…which is a different URL than the desktop sent me to, giving me “Vivaldi Social, powered by Mastodon, Log in with…[vivaldi.net button]” > click button and…
…nothing happens.
If I paste in the URL the desktop sent me to, in a new tab, that just redirects me to the auth/sign_in page, which doesn’t get me anywhere.

Brutaldon says "Not able to log in with this form? Maybe your brutaldon instance isn't visible on the internet to your Mastodon instance? If so, you can use the old login form." , so...

If I try
burtaldon.org/oldlogin
and input my details, I get
Invalid user name, password, or redirect_uris: ('Mastodon API returned error', 400, 'Bad Request', 'unsupported_grant_type')

C )
iPad iOS9.3.5(max), Brutaldon v2.15.0
Firefox or Safari
I go to https://brutaldon.org and click Log in , which takes me to
https://brutaldon.org/login
I enter…
https://social.vivaldi.net
…click Log in, takes me to
https://social.vivaldi.net/auth/sign_in
…which is completely blank apart from the words “log in” and, separately “vivaldi.net”. If I tap on the latter, it takes me to
https://social.vivaldi.net/auth/auth/openid_connect
…which gives me a “The page you are looking for isn’t here” error.

If I try
burtaldon.org/oldlogin
and input my details, I get
Invalid user name, password, or redirect_uris: ('Mastodon API returned error', 400, 'Bad Request', 'unsupported_grant_type')    

Now, I can live without option C, but as soon as social.vivaldi.net updates to Mastodon v5.0, I’m going to be completely locked out of Mastodon on my phone. (As I was on my iPad when Mastodon went from v3 to v4).
Hoping you can help.