Found a post about this problem dated in early February.
Just appeared for me this morning.
Resolved by turning off Avast web filters
Avast,
Do you have a fix?
Hi newmarket2,
what error message is displayed in browser when https site can not be loaded?
Can you please generate support package, send it to us and post here reply with ticket id?
https://support.avast.com/en-ww/article/Submit-support-file/
Thanks in advance,
HK
noting that the problem is frequent but intermittent.
Some https sites are not being blocked
Hi to all!!
I am experiencing the same issue here!
I have the latest Free Avast and the latest Firefox 65.0.2…
I did a clean install of the windows like a couple of days ago…Until today everything was fine, but now i have this message for ALL the websites i am trying to access with Firefox…“your connection is not secure” !! Even with trusted sites like adobe.com, gmail etc.
I attached an image!
Just to mention that I am using the same Windows 7 x64 ultimate for a lot of years (the same installation Cd), combined always with the free Avast and of course Firefox!! Never had an issue!
One more thing, i can easily go online with Chrome (that’s how i am writing this message!!)
FYI, I double checked the sources in the internet, so i found that if/when i disable Avast generally, or more specifically the Web Shield and the Mail Shield, I DO have access with Firefox again! But these are not desired solutions!!
I DID also check my PC for malware with avast, malwarebytes and spybot - search and destroy (i have been using these two extra tools - just for scanning and NO for real time protection - so many years with avast without any problem).
/edit: I also did repair Avast, nothing changed, later i did also re-install Avast, again, nothing!! the only thing that worked was disabling the shields…/
What should i do!?!? Could you provide any help?
Because the only solution i have in mind (except from disabling the shields which is not recommended or from stop using Firefox which it is not a desirable solution either cuz i truly like Firefox) is to find another antivirus (which, again, i don’t want to go to this direction, just because i also like avast and so many years we have been working great together 8) !!).
Thank you in advance for your time and effort…
Can you please submit the support package, as described above, and post its ID?
Thanks.
Although i am not sure about the info that this program is gathering… :o i just did it…
sorry can you please tell what to expect? regarding time etc.?
Why do you need someone to submit a “support package”?
This has been a known problem for about a month. It should be well on its way to repair.
I’m leaving my web filter off until we get a status of a fix
If this is the first anyone at Avast has heard of this, then
- you are doing a bad job of scanning for issues
- I need to consider switching from Avast
yeah you are right…silly me… this issue is been there for quite some years actually, as i found online… and i am asking what to expect regarding time… :S
No, this is certainly not the first time we’ve heard about this - on contrary; there has been quite some progress, program modules have been updated because of the issue - so yes, a new support package can indeed bring new information. Thanks for the submit, gkoletis1.
Another workaround should be released soon, in the next virus definition update.
Thank you for the reply!! 
So lets all wait and see… until then i will use Chrome instead…
Hi gkoletis1,
could you please try new beta version?
https://forum.avast.com/index.php?topic=225649.msg1496590#msg1496590
Beta already contains all necessary fixes that should prevent “your connection is not secure” error on loading https sites. If the problem persists even with beta please enable debug logging (Menu > Settings > General > Troubleshooting > Enable debug logging) and generate new support package. And also please post the error displayed in the browser after you click Advanced button?
Thanks in advance,
HK
Switching to the Beta version shouldn’t be taken lightly. Beta is not for the faint of heart.
Switching back also involves total removal of the beta version and a re-install of the regular version.
Hi there…
To tell you the truth, the next day everything worked fine, and still it is until now!!