You can use cafile (with the certificate bundle that you found), but it does something else and won’t help address this problem.
If you specify
certfile /etc/letsencrypt/live/cloud.letrikasol.com/cert.pem keyfile /etc/letsencrypt/live/cloud.letrikasol.com/privkey.pem
as you did before, I think mosquitto will be able to start running, but presumably serving an incomplete certificate chain, because we haven’t found a way to get it to serve the intermediate certificate.