Google Actions: Account linking sends GET request to token URL - actions-on-google

Question

I have some problems with the Account Linking for Google Actions:
I have implemented the OAuth2 steps described in the documentation by Google. I have implemented my OAuth2 server and tested it via Postman and am able to get an access token as expected.
If I try to authenticate from the Google Home app by adding the service to my account, I get taken to my authorization form, the authorization works fine and responds with an authorization code to Google's redirect URI as it is supposed to do. However Googles return URI says 'Account linking failed' and then I get a message in the Google Home app : 'Can't update the settings. Check your connection'.
Another strange thing that I've see from my logging of my Token URL script: I can see an incoming GET request from Google to my Token URL instead of a POST request with the required data as mentioned in the Google documentation. So even though the message 'Account linking failed' appears, it seems that Google is calling my token URL, but with a GET instead of a POST.
These are the logs of the requests to the token URL:
REQUEST FROM POSTMAN (testing software) > OK
__SERVER
Array
(
[USER] => www-data
[HOME] => /var/www
[HTTP_ACCEPT_ENCODING] => gzip, deflate
[HTTP_ACCEPT] => */*
[HTTP_USER_AGENT] => PostmanRuntime/7.6.1
[HTTP_POSTMAN_TOKEN] => f85664e2-7d38-4511-9519-cddda3feec06
[HTTP_CACHE_CONTROL] => no-cache
[HTTP_CONTENT_TYPE] => application/x-www-form-urlencoded
[HTTP_CONTENT_LENGTH] => 145
[HTTP_CONNECTION] => close
[HTTP_HOST] => 127.0.0.1
[REDIRECT_STATUS] => 200
[SERVER_NAME] => _
[SERVER_PORT] => 80
[SERVER_ADDR] => 127.0.0.1
[REMOTE_PORT] => 38622
[REMOTE_ADDR] => 127.0.0.1
[SERVER_SOFTWARE] => nginx/1.12.2
[GATEWAY_INTERFACE] => CGI/1.1
[REQUEST_SCHEME] => http
[SERVER_PROTOCOL] => HTTP/1.0
[DOCUMENT_ROOT] => [******]
[DOCUMENT_URI] => /google/token/index.php
[REQUEST_URI] => /google/token/
[SCRIPT_NAME] => /google/token/index.php
[CONTENT_LENGTH] => 145
[CONTENT_TYPE] => application/x-www-form-urlencoded
[REQUEST_METHOD] => POST
[QUERY_STRING] =>
[SCRIPT_FILENAME] => [******]
[FCGI_ROLE] => RESPONDER
[PHP_SELF] => /google/token/index.php
[REQUEST_TIME_FLOAT] => 1553765980.9273
[REQUEST_TIME] => 1553765980
)
__POST
Array
(
[client_id] => [******]
[client_secret] => [******]
[grant_type] => authorization_code
[code] => [******]
)
REQUEST RECEIVED WHEN TESTING WITH GOOGLE HOME APP on smartphone > NOT OK
__SERVER
Array
(
[USER] => www-data
[HOME] => /var/www
[HTTP_ACCEPT_ENCODING] => gzip,deflate,br
[HTTP_USER_AGENT] => OpenAuth
[HTTP_CONTENT_TYPE] => application/x-www-form-urlencoded
[HTTP_CONNECTION] => close
[HTTP_HOST] => 127.0.0.1
[REDIRECT_STATUS] => 200
[SERVER_NAME] => _
[SERVER_PORT] => 80
[SERVER_ADDR] => 127.0.0.1
[REMOTE_PORT] => 46184
[REMOTE_ADDR] => 127.0.0.1
[SERVER_SOFTWARE] => nginx/1.12.2
[GATEWAY_INTERFACE] => CGI/1.1
[REQUEST_SCHEME] => http
[SERVER_PROTOCOL] => HTTP/1.0
[DOCUMENT_ROOT] => [******]
[DOCUMENT_URI] => /google/token/index.php
[REQUEST_URI] => /google/token/
[SCRIPT_NAME] => /google/token/index.php
[CONTENT_LENGTH] =>
[CONTENT_TYPE] => application/x-www-form-urlencoded
[REQUEST_METHOD] => GET
[QUERY_STRING] =>
[SCRIPT_FILENAME] => [******]
[FCGI_ROLE] => RESPONDER
[PHP_SELF] => /google/token/index.php
[REQUEST_TIME_FLOAT] => 1553767309.7797
[REQUEST_TIME] => 1553767309
)
__REQUEST
Array
(
)
__POST
Array
(
)
__GET
Array
(
)
-------------------------
__ANSWER
400: invalid grant
Configuration in Actions Console

Answer 1

Problem seemed to be caused by the redirect of / to /index.php without the POST values.
Fixed it by changing my URLs in the Actions Console to /index.php and now the linking works fine.