Hi, I have problem with synchronize (/identities//synchronize.json) for Twitter. For Facebook for the same user (API User Token) it works, but for Twitter oneall returns:
{
  "response": {
    "request": {
      "date": "Fri, 08 Jul 2016 09:24:15 +0200",
      "resource": "/identities/XXXXXXXXXXXXXX.json",
      "status": {
        "flag": "error",
        "code": 400,
        "info": "The provider did not accept the stored access_token, it is either invalid or expired. Please ask the user to re-login."
      }
    }
  }
}
I have connected Facebook and Twitter for that user. 
I tried to include some POST data - "update_user_data" and "force_token_update" but still same result.                
                             
        
Answers
A quick test with update_user_data == false, and force_token_update == true, for a Twitter identity token, worked.
However, the this was right after logging in.
What was your scenario?