Dropbox sign refresh token getting expire after 1 hour

Did you get a chance to read over the documentation for Dropbox Sign's OAuth workflow?

Only reason I'm asking is because the samples you provided include endpoints not related to Dropbox Sign API, with parameters that seem to be for Dropbox API's OAuth workflow and not Dropbox Sign's.

For example, your authUrl : 

authUrl='https://app.hellosign.com/oauth/authorize?response_type=code&client_id=12345678&redirect_uri=redirect-url&token_access_type=offline&state=121324234'

is using endpoint /oauth/authorize which is not an endpoint for Dropbox Sign API. Also, the query parameters are not relevant to Dropbox Sign — except for client_id and state.

Additionally, the body you are sending on your request:

--data '{
    "code": "code",
    "grant_type": "authorization_code",
    "redirect_uri": "redirect_uri",
    "client_id": "client_id",
    "client_secret": "client_secret"
}'

 is missing the state parameter. Also redirect_uri is not relevant to Dropbox Sign.

I would recommend attempting the following cURL command to generate an access token:

curl -X POST 'https://app.hellosign.com/oauth/token' \
  -u 'YOUR_API_KEY:' \
  -F 'client_id=YOUR_CLIENT_ID' \
  -F 'state=state' \
  -F 'code=code' \
  -F 'grant_type=authorization_code' \
  -F 'client_secret=client_secret'

And the following to generate a new access token, when needed:

curl -X POST 'https://app.hellosign.com/oauth/token?refresh' \
  -u 'YOUR_API_KEY:' \
  -F 'grant_type=refresh_token' \
  -F 'refresh_token=refresh_token'

If you are still receiving errors from Sign's API OAuth endpoints. Go ahead and contact our support team, from the email you are using to test Dropbox Sign's API, so they can further troubleshoot.