Can't log-in to Fever API from ReadKit

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I am using the public https://www.commafeed.com, not a self-hosted instance.

I can log-in to the Fever API just fine using Unread using:

API endpoint: https://www.commafeed.com/rest/fever/user/27113221
e-mail/username: aram
password: API_KEY

I want to use a different client though, and the same settings do not work in ReadKit. App reports "Invalid credentials". AFAIK there is no way to enable client-side logging in the app.

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From Fiery Fields (another client) I get "Not authorized" with the same settings.