LinkedIn: Unable to retrieve access token

Hello.

After set up my LinkdIn App with the correct Client ID and Client Secret and authorizing my LinkedIn Account (without Marketing API) I get the following error:

[error] => invalid_request
[error_description] => Unable to retrieve access token: appid/redirect uri/code verifier does not match authorization code. Or authorization code expired. Or external member binding exists

Also if I try to submit a test post I get this error:

Array
(
    [postID] => 
    [isPosted] => 0
    [pDate] => 2019-06-26 11:54:30
    [Error] => Array
(
    [serviceErrorCode] => 65600
    [message] => Invalid access token
    [status] => 401
)

)
Array ( [postID] => [isPosted] => 0 [pDate] => 2019-06-26 11:54:30 [Error] => Array ( [serviceErrorCode] => 65600 [message] => Invalid access token [status] => 401 ) )

Do you have any ideas, what the problem could be?

Kind regards