After upgrading to new computer with Ubuntu 20.04 I managed to see such error “1 signature is unknown!”.
SignatureXAdES_LTA.cpp:198 Signature validation
SignatureXAdES_B.cpp:738 Unable to verify signing certificate
X509CertStore.cpp:351 unable to get local issuer certificate
OCSP.cpp:308 Certificate status: unknown
Issue happens because openssl version 1.1.1f that is included in Ubuntu 20.04 LTS https://github.com/open-eid/libdigidocpp/issues/341
Building openssl 1.1.1g from source helped to fix that issue. However during writing this post there are still some certificate validation issues open with this version.
One guide to help you build openssl from source can be found from here https://www.howtoforge.com/tutorial/how-to-install-openssl-from-source-on-linux/