(OOps, posted this in the WUProp What's going on. Meant to post it here.)

I had trouble with collatz and did some forum browsing.
A hand full of projects use a security certificate that has recently expired.
The latinsquares forum also talks about the certificate issue.

Options are finding an updated file called:

ca-bundle.crt

And replacing the one on your machine in the ProgramFiles/BOINC directory.

The other option, which I used, was to edit the ca-bundle.crt file, removing the offending certificate code.
This worked for me.

The code is a block in the ca-bundle.crt file labeled: "DST Root CA X3"
I deleted that line and all text below it that was between and including the "-----BEGIN CERTIFICATE-----"
and "-----END CERTIFICATE-----" lines.

I then stopped and restarted BOINC Manager.

(I also made a copy of the file before I edited it in case I messed things up and needed to put it back.

Supposedly, the next release of BOINC manager will install an updated ca-bundle.crt file with newer certificates.