I get always the error: "invalid client_id or redirect_uri" from signin authorize endpoint.

[Saruman]

Hello Community,

I want to develop a APP for managing user transactions. The user should be able to login by his PayPal account

https://www.paypal.com/signin/authorize?...

to get a authorization code. In the next step I should get the access token from 

https://api.sandbox.paypal.com/v1/oauth2/token

For example: 

https://www.paypal.com/signin/authorize?response_type=code&scope=openid+profile+email+transactions&c...

But I get always the error: invalid client_id or redirect_uri

In my sandbox the redirect URI is added and is exactly the same as in the request. My sandbox client ID is also correct set.

What wents here wrong?