Bingo! That's it. After my last comment and before yours, I did start recalling about that avast forum hacking attack and that it went off line for sometime and then came back eventually. Yes, shortly after the forums' comeback whereby of course there had been changes made to the log in system among other areas ... that's when I then lost the ability to log in. I just didn't remember the timeframe of when that was. I figured if I said that it was over a year ago and then you went and checked and saw comments of mine way after that, I'd look bad.
Okay, the answers:
(1) When I use my sister's High Speed Internet at her house, I use HER laptop with currently Windows 10. The last time I logged in into the avast forums with her laptop, it had Windows 7.
(2) As far as another OS that I might have logged on to the avast forums with my Dial Up here? If the avast forums hacking attack was in 2014, then Windows XP would be the only OS with which I have attempted to log on during this failure period. Because I did have a computer with Windows Vista for about a year, but that was before my previous computer (Also with Windows XP), which I recall getting in March 2013. So ONLY Windows XP. Just keep in mind ... I WAS able to always log on with Windows XP before the hacking attack fix to the forums.
(3) I don't use my computer for any secure HTTPS sites like banking sites, Etc. But, if NON-banking type HTTPS sites count in the course of your question, then I AM able to log on into other regular HTTPS sites.
(4) No, I have never made use of those Privacy Tabs / Options on browsers. I guess I could try it a bit later this afternoon to see what happens and let you know.
(5) Oh yes, I'm able to access / view the avast forums without any problem.
(6) I have always used Modem PCBs inside whatever computer I've had. So as for powering them down and back up and restarting them, that happens everyday.
(7) As for clearing out the browser cache, temp files, private data, history, etc., that has happened an ultra mega thorough gazillion times during this failure to log on into the avast forums period. I frequently clear all that stuff with CCleaner, Glary Disk Cleaner, Glary Tracks Eraser, Glary Utilities Slim, Wise Disk Cleaner, Wise Registry Cleaner, JetClean and the Browser Clearing Functions. During this log in failure period, the Browsers have even been uninstalled and reinstalled many times.