Validation error, please try again. If this error persists, please contact the site administrator
The form token associated to forms created with the form API is a value that is used to verify the data are submitted from a user who filled a module form, and not forged data.  

The drupal_valid_token() function that validates the token is validating it using the user session; this means that, if the user session used when the data are being submitted is different from the session used when the form was created, the validation of the token fails.
This could happen also when a module calls session_destroy().