Site blocked due to invalid signature

Access to several web sites is being blocked by Avast and pop-up indicates that the digital signature is invalid.
As these are national companies web sites and https, is this really likely?
How can I overcome this problem, other than by disabling Avast?
Thanks for any help offerred.

I had the same problem and found this cleared it up : open Avast, click Settings then choose Active Protection. Click customize on Web Shield. Now un-chech the box next to Enable HTTPS Scanning. That’s it. Happy webbing!

Hi, are you guys running Windows XP?

Thanks.
That did enable me to access the sites.
Seems the “wrong” way to resolve the problem though, however it will suffice.

Was using XP

Hey, Just wanted to let you know that I updated my program (still free) to 11.1.2241 today and went back in settings and re-enabled the HTTPS Scanning and all is well. The sights that were getting messages of invalid certificates before are now all coming up with no problems. I am also using XP sp3.

https scanning has changed in the latest version (2016).
Update (or do a clean installation) and the problem should be solved.