Hi Polonus.
These are the rest of the directions, based on Milos' instruction:
Left click on the avast! icon in your system tray.
Click on Open Avast user interface.
At the bottom of the new menu, click on Settings.
At the top of the new menu, click on Active Protection.
In the Active Protection window, go down to the "Web Shield" line and click on Customize, which opens the Web Shield Settings / Main Settings window.
Uncheck "Enable HTTPS scanning" and close and exit out of the bloated avast! interface.
Firefox has an add-on called Calomel SSL Validation, which will give you a summary and "security grade" for the webpage that you're viewing. The one that avast! was blocking until I disabled the HTTPS Scanning is rated by Calomel "93%," which is pretty high, and says they're using TLS 1.2, which is the most modern security available for webpages (or something like that, see
https://en.wikipedia.org/wiki/Transport_Layer_Security).
So, avast! was blocking a webpage even though it had the latest security. Milos says it was XP's fault for being too old to read modern certificates. If that's the case, then I suppose all antivirus programs will have the same problem on our XP machines when they see these new certificates.
In summary, Polonus, I hope you and I are both looking at strategies for updating our computers to a modern OS. Recommend checking out
http://distrowatch.com/dwres.php?resource=major and
http://distrowatch.com/ For upgrading our computers, the most important thing is backing up our data files in one or two places. And take a screen shot or write down the list of all the programs you have installed on XP. Then when you get a new operating system, you will have all your data files, and you can re-install the programs that you had on XP, or close approximations thereof. And program licenses will also need to be backed up. Firefox bookmarks, at least, should be backed up and exported to a file.
I know you never said you have Firefox, but I like saying its name.
If you're rich, you can always get a new computer, and keep the old computer running until you're sure you've gotten all the data and information off of it that you'll need. At that point, I would say that AMD and core-2-duo-and-older computers should be electronics-recycled. i3, i5, and i7 computers should be shined up, brought flowers, and allowed at the dinner table and under the bed covers.
Best luck.
Thanks to Milos for pointing out the offending checkbox. Google went right to him.