This one was a bit weird. I run BOINC on one machine and all of a sudden WCG stopped uploading with things like..
[World Community Grid] Scheduler request failed: Peer certificate cannot be authenticated with known CA certificates
or another generic one about SSL error.
Turns out playing with some flags (Gentoo build controls options) I had changed curl from using OpenSSL to GnuTLS. No biggie you would have thought but it just does not work for WCG.
Recompile it using OpenSSL makes it all happy again. So if you see these errors see if your curl has openSSL listed after it when you start the boinc_client.
This is all for Linux of course :-)