Looks like a lot of people have problems with Avast on Windows 10 (and other).
Solution to certificates problems is described on this avast fansite.
They say:Solution/Fix for the Invalid Certificate Issue Caused by the HTTPS Scan
Recently lot of Avast users (especially the ones running Windows XP) are experienced an issue when Avast has been blocking several major websites because of the invalid cloudflaressl.com certificate. The full message on the pop-up is ‘Avast Web Shield has blocked access to this page because the following certificate is invalid: [name of the certificate]‘.
To fix this try one of the following actions:
- Update your Avast to latest version available
- Disable HTTPS scanning by going to settings » Active Protection » Web Shield » Customize » Main settings » Un-check ‘Enable HTTPS scanning‘ and restart your computer