I am having an issue obtaining an access token using JWT too. I am getting "JWT token is incorrectly formatted, and can not be decoded" error whatever JWT token I use.
Here is an example request I am doing

curl -X POST -H "Content-Type: application/x-www-form-urlencoded" -H "Cache-Control: no-cache" -d 'client_id=***&client_secret=***&jwt_token=eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.EkN-DOsnsuRjRO6BxXemmJDm3HbxrbRzXglbN2S4sOkopdU4IsDxTI8jO19W_A4K8ZPJijNLis4EZsHeY559a4DFOd50_OqgHGuERTqYZyuhtF39yxJPAjUESwxk2J5k_4zM3O-vtd1Ghyo4IbqKKSy6J9mTniYJPenn5-HIirE' "https://ims-na1.adobelogin.com/ims/exchange/jwt"

client_id and client_secret are obfuscated here, but they are correct when I'm getting the error since omitting or tampering one of them will result in a "client_id/client_secret are incorrect" error instead.
Concerning the jwt_token, I tried various token, I tried base64 encoding the all token, etc. It is always returning the same "JWT token is incorrectly formatted" error. In the example, I used a JWT example from https://jwt.io/#debugger, so it is correctly formatted and can be decoded.
I am not sure how is supposed to be formatted the jwt_token parameter value.