I have generated a self-signed certificate for personal use -- and maybe my problem is that the Squid version I use is somewhat older -- but the certificates that my Squid proxy server presents to Opera and that are signed by my "root authority" (it is a root authority in my world) are seen as invalid.

I might try upgrading my Squid version first but that's hard on Linux. Might do.

The self-signed was generated by using openssl:

openssl req -new -newkey rsa:1024 -days 365 -nodes -x509 -keyout <out.key> -out <out.pem>

And then I filled in almost every field that was (required) for the certificate. I don't know why this Opera won't accept them. Firefox accepts them just fine.

The new self-signed was imported into Opera as a root CA without issue. It is only the certificates that are now presented to it by Squid are seen as "invalid" without any reason given.