Also it's possible that the IP address your script is connecting from is not correct (set in your Dynadot account: https://www.dynadot.com/account/domain/setting/api.html) The system may respond with a message like that but you need to check the error details...
That error message looks like a search command error. Do you have command=register in your url? The register request example was using 'search', but it has been changed to 'register'.