For some time now, there have been problems with long-term validations of short-term certificates with the RTR Validator. swisscom is already in discussions with RTR to fix the problem. (Source: https://trustservices.swisscom.com/status/24-04-2023-der-rtr-validator-gibt-nicht-das-korrekte-ergebnis-zurueck-wir-empfehlen-den-eu-validator/ ).

We therefore recommend – as do RTR and swisscom – to use the EU Validator under https://ec.europa.eu/digital-building-blocks/DSS/webapp-demo/validation

Please note that only qualified electronic signatures can be verified with the validator. Advanced electronic signatures, such as “sproof standard” or other non-qualified signatures, cannot be validated there. However, you can upload a document to sproof and use the validator built in there for verification.