Before switching from N9 to Sailfish X, I was having the same problems you describe. I was able to resolve this for some of the main sites I visited.

I figured the problem may be that since Nokia isn't providing updates to the N9, perhaps it's not getting updated root certificates and that is causing a problem. So did a search one day for intermediate certificates on a PC, downloaded them to the PC. I then hooked my N9 up to the PC and copied over the files.

Next, on the N9 I went to Settings>Security>Certificates. Then in the hamburger menu at the bottom right, I selected "Add new certificate".

Once I imported the correct intermediate certificate, I tried the website. Like I said, for some it resolved the issue.

I can successfully open openrepos.net on my N9. I tried seeing what SSL intermediate would be used to solve openrepos.net and I can't figure it out now.

I know I imported four of the Comodo ones, I think a handful of GeoTrust ones, and GoDaddy's Root CA.

I am sorry this isn't more specific as it has been quite a while since I did this. But I hope perhaps it sparks an idea to get you up and running.

If you want more help, perhaps tell me a site that isn't working for you. I'll see if it's not working on my N9, then see if I can get it to work and keep track of exactly what I download from which site so you can replicate the successful procedure.

EDIT: Ah, I reread the thread and see Twitch listed as a non-working site. That does not open on my N9, either. I am trying with that site now.