Thanks for responding. I wondered about that because of the ereservations.att.com web site that was listed above. The situation has cleared up. If it occurs again, I will go straight to ATT.

We have DSL with AT&T. We use a router to which a few computers are linked. We don't use a VPN, if that is what you mean by a proxy.

@leocg

Thanks for responding.
I use Malwarebytes and Avast.
I received no warning from Avast while I was trying to access any web sites.
When I read you entry here, I ran Malwarebytes and came up clean.
There had been no change in any of my software settings in a long time.

Nobody have any ideas or suggestions?

I use Opera 63.0 with HTTPS installed. My computer is a Mac Mini using OSX 10.11.6 (El Capitan).

Something called HSTS suddenly started blocking access to many web sites today — Thursday, October 3 — with no prior warning or indication or anything. I am now unable to access some of my most important and most frequently accessed sites.
Can anyone help me?

I get this instead:

Or I get this:

@avl

Opera 50 crashed frequently, with no discernible pattern. In addition to that, the extension HTTPS Everywhere -- downloaded from Electronic Frontier Foundation -- which I had used in earlier versions of Opera, started going bad in Opera 50. Two or three times.
I just downloaded Opera 51. It has already crashed once. So we'll see. If it crashes again, I'll return to Opera 49 and try disabling auto-update.
If that doesn't work, I'll have to move on from Opera.
Thanks for the input.

So if Opera 50 is unstable and keeps crashing, and returning to an earlier version is at best a short-term solution, what's left. It seems I am then limited to Safari and searching for a new browser. Any suggestions?

@bobamamda-0

Is there nothing? If I simply drag Opera 50 to the trash and run the installer for Opera 49, what happens then?

Mac mini (Late 2014)
processor: 1.4 GHz Core i5
Memory: 4 GB 1600 MHz DOR 3
OS X 10.11.6
Opera 50.0.2762.67

I just upgraded from Opera 50 -- which kept crashing -- to Opera 51. And all my bookmarks are gone. Several YEARS worth of bookmarks are gone. This has never ever happened with any previous upgrade and I've been using Opera for more than four years. Is there ANYTHING I can do or am I totally screwed?

Mac mini (Late 2014)
processor: 1.4 GHz Core i5
Memory: 4 GB 1600 MHz DOR 3
OS X 10.11.6
Opera 50.0.2762.67

Opera 50 keeps crashing to the extent it has become unusable. I never had this problem with an earlier version of Opera. I still have the installers for Opera 43 & 49. I would like to re-install 49 (unless there is a reason 43 would be better?) but I'm not sure what to do. Do I simply drag 50 to the trash and run the installer for 49? I've never tried replacing a newer version of an application with an older version when there wasn't an uninstaller available.