Adobe reports the error that the signature is invalid because it could not be validated.

“Signature is valid, but the signer’s identity validity could not be verified” is Adobe’s statement if no long-term validation format (LTV) was used with an electronic signature. A long-term validation format stores the validity information at the time of the signature. The reason is that Adobe tries to check the validity of a 10-minute certificate. If you did not use the long-term validation format for electronic signature, Adobe could no longer access the signature after some time. Therefore, signatures with short-term certificates (but also signatures with long-term proof) must always be saved in LTV format. You can find more information on our GitHub: 

https://github.com/SwisscomTrustServices/AIS/wiki/PAdES-Long-Term-Validation