u2f-php checks the certificate of a yubikey by default - but newer devices (a YubiKey 5 NFC) seem not to use the yubico certificate form 2014 (https://developers.yubico.com/U2F/yubico-u2f-ca-certs.txt). While https://www.yubico.com/genuine/ verifies the yubikey, u2f-php says "Signature verification failed". With older yubikeys (YubiKey NEO) everything works fine, so I assume, it might be a certificate issue.
Unfortunately, I don't find any good starting point for debugging this issue. Any ideas?