@monocles I'm using a pretty recent set of ciphers, but the problem was solved when I issued a new cert, now from Let's Encrypt.
Ciphers: ECDH+AESGCM:EDH+AESGCM:ECHACHA20+POLY1305:ECDHE+CHACHA20:ECDHE+AES:@STRENGTH:!SHA:!CBC
Curve: secp256r1
Min TLS version: 1.2