Hi @fairose,
I still use the way mentioned in my post from almost 5 years ago. It gets overwritten from time to time, but I just re-run the commands to trust my cert again.
I think I changed .pem to .crt, but otherwise it is in /usr/share/ca-certificates/certificate.crt (filename doesn't really matter).
I don't know why are you getting the error, but I guess there is a problem with some certificate you put in there.