Error while generating token

I just tried out this flow and it seems to be working correctly for me. Here's a sample of what I ran (with values redacted):

# https://www.dropbox.com/oauth2/authorize?client_id=APPKEY&response_type=code&token_access_type=offline

curl https://api.dropbox.com/oauth2/token \
    -d code=AUTHORIZATIONCODE \
    -d grant_type=authorization_code \
    -d client_id=APPKEY \
    -d client_secret=APPSECRET

# {
#   "access_token": "ACCESSTOKEN",
#   "token_type": "bearer",
#   "expires_in": 14400,
#   "refresh_token": "RERESHTOKEN",
#   "uid": "USERID",
#   "account_id": "ACCOUNTID"
# }

curl https://api.dropbox.com/oauth2/token \
    -d refresh_token=RERESHTOKEN \
    -d grant_type=refresh_token \
    -d client_id=APPKEY \
    -d client_secret=APPSECRET

# {
#   "access_token": "NEWACCESSTOKEN",
#   "token_type": "bearer",
#   "expires_in": 14400
# }

If that isn't working properly for you, please share the steps and code to reproduce the issue so we can look into it.