Just a follow up on this, since I have now found a solution that works for me. I first created a free ZeroSSL certificate on zerossl.com. The content of the files certificate, ca_bundle and private I pasted in the fields at Add SSL in CyberPanel. The ZeroSSL certificate was installed instead. After that I go back to CyberPanel and issued a new Lets Encrypt certificate, and this time it was installed perfectly and overwrite the ZeroSSL certificate.