API always return 6500 - Internal Server Error while processing this request

API always return 6500 - Internal Server Error while processing this request

I've register and get both API and ticket id. However, any calls from get login id to projects task list, I got same error:

<response uri="/api/private/xml/dashbs/getlogin"><error><code>6500</code><message>Internal Server Error while processing this request</message></error></response>

Is there anyone can help ? I've tried to change the api and ticket id to a wrong one and the system return a wrong api and ticket id message with this one. So, it should be the API and ticket id.