I have spent several hours with the above error. Even Alexa developer support only redirects you to the FAQ here.
The error in my case was caused by an IPv6 only OAuth server. After Alexa successfully redirects to the login page of the OAuth server (this is only a redirection to a web page) and you successfully log in, a one time key is issued and forwarded to the response URL of the AWS Cloud.
When the token is retrieved, the error occurs - Alexa cannot retrieve a token because the server cannot access the IPv6 URL of the OAuth server.
BTW: No log entry on AWS side !!! On the OAuth server everything logged successfully, just never retrieved the token!
Solution: I now use an HTTPS IPv4 => IPv6 proxy in front of the OAuth server.