The error received from the web-based validator is related to the signature error category, which is due to the difference between the X.509 certificate (PCSID) that you have signed your simplified e-invoice with and the x.509 certificate that is being used in the validation.
The web-based validator has a default certificate in the back-end which differs from the one that you obtained during the API onboarding process, so receiving a signature error while validating a signed simplified tax e-invoice using the web-based validator is a normal behavior, so please don’t worry about this specific error & rely on the API response