also, (though this won’t be the cause of your problem right now) use the ones from the /etc/letsencrypt/live/ directory, not the archive, as the live ones will be updated when you renew (which you’ll be doing every few months, hopefully automatically)
I suspect you’re going to have better luck talking to libwebsocket people about your problem and coming back only in the unlikely event that there’s a problem specifically with the Let’s Encrypt certificates.