For some reason OpenSSL we use does not find the root certificate for secure.thisistrue.com
(Go Daddy Class 2 Certification Authority) even it is in the certificate file the webshield uses.

openssl s_client -CAfile /Library/Application\ Support/Avast/config/com.avast.proxy.cer -connect secure.thisistrue.com:443

will give you the same error.

I will try to investigate it a little bit more.