Omniture.authenticate Invalid User specified error

Hi, 
I've been using this library for a while now. But since yesterday I ran into a new issue while running the below line : 
omniture.authenticate('my_username', 'my_secret')

My username and my secret are still the same but an error message says "omniture.reports.InvalidReportError: Bad Request: Invalid user specified."

How can I fix this error ?
thanks