This was discussed in another thread, and it just occurred to me I should post a news item with more explicit instructions.

The issue is that the BOINC client comes bundled with SSL certificates and these expired earlier today. Here are the basic steps to fix the problem. (Note: this works on linux and I see no reason why it shouldn't work on Windows too.)
1. Download the new "ca-bundle.crt" file from here:
https://1drv.ms/u/s!AsVDg7OAm7-whqEq...UoBKA?e=VHwBAP
2. Replace the old ca-bundle.crt file with the new one (it should be in your BOINC root directory).

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