This error can occur after too many login attempts. The Toodledo API service limits the number of authentication requests within a set period of time (Each user is allowed to request 10 authentication tokens per hour).
If too many logins are requested then the account is locked from the API service. This can occur after multiple successive login/logout attempts, or if an API errors cause the authentication token to expire.
The user must wait until the API authentication lock out expires, typically after 1 hour.