Haven’t tried Warm Spare on v15 yet. I don’t see anywhere mentioned that the PBX needs a non-self signed certificate.
From the error link:
CURLE_PEER_FAILED_VERIFICATION (60)
The remote server’s SSL certificate or SSH md5 fingerprint was deemed not OK. This error code has been unified with CURLE_SSL_CACERT since 7.62.0. Its previous value was 51