Some of you might be experiencing an error "Certificate has expired" when synchronising with Joplin Cloud (and possibly other services) when using the desktop application.
This is due to Let's Encrypt root certificate that expired on 30 September, and the new method they are using is not compatible with the Joplin desktop application.
This actually affects thousands of applications, not just Joplin, so various solutions are being considered right now and hopefully a fix will be available relatively soon.
For now, as a workaround, you can simply check "Ignore TLS certificate errors" in Configuration > Synchronisation > Advanced Options
I will let you know as soon as a fix is available so that you can clear that option.
More info:
Issue with Electron and expired root on Let's Encrypt
Let's Encrypt root CA isn't working properly on Electron GitHub repository
Update: I have implemented a temporary fix on Joplin Cloud which should solve the issue for now. If you're still having some issues please let me know. An updated desktop app will be available later on with a more permanent fix.