Same thing for me after the latest update, and as described above, turning hardware acceleration back ON made it go away. Bit wary of what is actually going on in the background though, for this to just start outta nowhere.

Today's update has returned things to normal, thankfully.

Also happening here, 46.0.2597.32 on Windows 7. Very annoying.

That would be annoying, so I checked to see for myself. These are my findings.
Opera 43.0.2442.991 (64-bit) on Windows 10 still offers a dialog box for:

Incorrect,
Self-signed,
Expired certificates.

See these test links: https://onlinessl.netlock.hu/en/test-center/invalid-ssl-certificate.html.
However, for hash functions that are no longer considered safe e.g. expired SHA-1 certificates, Opera 43 no longer offers you an option to continue. See this test link: https://expired.identrustssl.com
Hope this helps.

I hadn't bothered working out the exact problem causing it, thanks.
In the end, they're gonna make the decisions they feel work best for the majority of people.
I'll live.

As a followup, it's slightly amusing that the "concept" browser Opera Neon does the correct (previous) thing and lets the user continue anyway, while standard opera now does not. Almost makes a person wonder if this change was actually a bug rather than intentional.

Anyhow, Vivaldi behaves properly as well, so perhaps it's time to make the move over there. It seems to have stabilized quite a bit since release. Sad, because I've been an Opera fan since almost the beginning.

It's not that the problem is big, it's that something which was previously up to the user is now decided by someone else.
They've taken away the choice completely, for no real apparent reason.
There are certainly tons of ways to work around it, but it speaks to a larger mentality at the company that users must be protected from themselves.

Uhm, what kind of Opera version are you guys actually using? I'm using the latest Opera 43 (43.0.2442.806) and I have no issues with this. So I think either something changed (doubtful considering the short timespan) or you guys are talking a lot of nonsense up there:

So yeah...

In cases where the certificate has incorrect information, the popup shows as usual. In cases where the certificate has expired, the popup shows as usual. It's only when both those things are the case where, for some reason, the popup does not appear and you cannot proceed. Presumably it is to protect average users from malicious sites.
All I'd like is some sort of internal flag I can check to override this behaviour, but according to the support rep this behaviour is intentional and not going to change.

I spoke with an opera rep on Twitter and the response was essentially "Okay, well, we've decided to keep it this way." Very unfortunate.

Yes, I had to start using Chrome for my dev tasks. It's quite frustrating.

Prior to updating to version 43, if you went to a site that had an expired or invalid cert you could click "Continue anyway" on a dialog box and just keep going. As of updating just now, this is no longer possible, and you just get a "you can't proceed" message.
Is there any way around this? Why was this changed?
As a web developer, it's often necessary to work on test/staging environments with incorrect or outdated certificates.
I really don't relish the idea of having to use a separate browser for work.