Comment 58 for bug 246205

Ok, the reason why it works with ca-bundle.crt removed is that boinc/curl uses /etc/ssl/certs/ca-certificates.crt then by default.

You can test whether you have the required cert installed using the following command (in a shell):
  curl -s -I https://secure.worldcommunitygrid.org/ > /dev/null ; echo $?
It should say "0".

The required certs appear to be:
mozilla/Entrust.net_Secure_Server_CA.crt
mozilla/Entrust_Root_Certification_Authority.crt

Please check with "sudo dpkg-reconfigure ca-certificates" if you have those enabled.

Also, make sure that you have no stale /var/lib/boinc-client/ca-bundle.crt file lying around (it should be a symlink - or you could remove it). You could check this using:
  curl -s --capath /var/lib/boinc-client/ --cacert ca-bundle.crt -I https://secure.worldcommunitygrid.org/ > /dev/null ; echo $?

Please provide feedback here, if you are experiencing the problem currently, and include your distribution, if you had/have the certs installed (now) and how the "curl" commands mentioned above fail.