Check this out for solution: https://github.com/SynoCommunity/spksrc/issues/4897
Essentially this commands can fix it:
# ssh to your NAS first
sudo -i
mv /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt.bak
curl -Lko /etc/ssl/certs/ca-certificates.crt https://curl.se/ca/cacert.pem