When accessing with Cl@ve this error appears:

Error (no. message.validation.error.code) processing request: message.validation.error.code - Assertion is null, empty or the response is encrypted and decryption is not active.

This error is returned by the Cl@ve platform and indicates a problem with the credentials.

If you are accessing with a certificate/DNIe, please verify that you are using a valid certificate.

If you are accessing with Cl@veMovil, please note that in this case the credentials have an associated security level depending on how they were registered in the service. To modify the security level of your Cl@ve credentials, follow the instructions on this page:

https://sede.agenciatributaria.gob.es/Sede/ayuda/consultas-informaticas/firma-digital-sistema-clave-pin-tecnica/modificar-datos-renunciar-servicio-clave/obtener-nivel-seguridad-superior-certificado.html

Please note that with Cl@veMovil you can access the headquarters but you will not be able to perform electronic signing if the procedure requires it. If this is the case, check the requirements to carry out the signing.