Service API acting strange with Answers module
I've figured it out, basically the answers module was changing the submit form's submit button text value. Doing this would break Service and make it not add any node to the database.
The solution for me was to remove that modification made by answers module by commenting out some lines:

answers.module , line 163
// $form["actions"]["submit"]["#value"] = t("Post your answer");


And everything works fine after saving.