@slartyblartfast According to openssl the Vivaldi Sync server certificate is valid
$ openssl s_client -connect bifrost.vivaldi.com:443 | openssl x509 -dates -noout
depth=2 C = US, O = Internet Security Research Group, CN = ISRG Root X1
verify return:1
depth=1 C = US, O = Let's Encrypt, CN = R3
verify return:1
depth=0 CN = bifrost.vivaldi.com
verify return:1
notBefore=Jan 8 05:50:29 2022 GMT
notAfter=Apr 8 05:50:28 2022 GMT