"Failed to get authorize url" when I'm trying to add / log in with my Misskey instance

[yitsushi]

Description

When I try to add my Misskey instance, Search is able to find the instance but when I click on the Login button it pops up an error message "Failed to get authorize url".

The Misskey App is created in the database, so it was able to create the app and get the secret for it:

misskey=> select "id", "name", "createdAt", "permission", "description", "userId", "callbackUrl" from app where name = 'Whalebird';
-[ RECORD 1 ]----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
id          | 85aylgrfqj
name        | Whalebird
createdAt   | 2020-03-25 13:59:54.651+00
permission  | {read:account,write:account,read:blocks,write:blocks,read:drive,write:drive,read:favorites,write:favorites,read:following,write:following,read:mutes,write:mutes,write:notes,read:notifications,write:notifications,read:reactions,write:reactions,write:votes}
description |
userId      |
callbackUrl | https://slippy.xyz

In access.log I see:

1. It hits GET /api/v1/instance and gets 404
2. It hits POST /api/meta and gets 200
3. It hits POST /api/app/create and gets 200 (user agent: axios/0.19.1)

After that I can't see any relevant requests.

How To Reproduce

1. Add Instance button
2. Fill in slippy.xyz
3. Press Search
4. Press Login

Your Environment

• OS: Windows (I tried the MacOS version too, same results)
• Whalebird Version: 4.0.0
• Instance: slippy.xyz