https://bitcointalksearch.org/topic/this-connection-is-untrusted-1053184
I locked the thread because I thought it was probably just a problem on my side rather than the forum itself. Nobody else other than one other person (Operatr) had experienced the same problem.
I was not using Tor or a VPN. I was using a fresh install of Portable Firefox.
Hope this helps.
So then what would it most probably be if it's not a result of an exit node? I also got this but on my tablet.
It can't be, since I wasn't using Tor.
Nevertheless, it seems to have been a transient problem (although a rather odd one I admit). Perhaps whoever supplies the SSL certificate for Bitcointalk had some issues which have now been resolved. I personally wouldn't be too worried about it.