General Error in API

General Error in API

Hello, I'm trying to get a Refresh Token via the zoho API.
I followed the documentation but all I get is a general error.

What I did:

1. Create Self Client in zoho Developer Console
2. Copy Client ID/Client Secret
3. Created a code with the needed scope (valid for 10 minutes)
4. Create the following request in Postman:

I also tried to use x-www-form-urlencoded with the same result.
If the code gets invalid after 10 Minutes, I get a "Invalid Code" error, so something is working here.
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