Prior to Win 8.1/64 bit to WIn 10 update no problems
Now sending email generates this certificate error. Your website says to check usiung certmgr.msc.
I can confirm that there is a cert registerd in Trusted Root Certification Authorities/Certificates - it has validity dates 01/01/2010 to 01/01/2040.
Running wscert.der as recommended generates a new cert with these dates - I did not register this new cert.
Viewing the details of the cert reported in the error message this has dates 07/01/1016 to 06/04/2016
3 images attached
1 - Error message
2 - details of cert causing error
3 - details of cert already registered im windows, which is same as generating a new one
Before uninstalling double checked with certmgr.msc - there are 2 avast certs registered. Uninstalled using the normal control panel/programmes and features method. Rechecked using certmgr and there was stillan Avast cert registered. It had 2014 dates. Removed this manually using certmgr.
Then downloaded and installed a fresh copy of Avast free version.