nuki_hub: Cannot use let's encrypt certificate

I’m having trouble enabling TLS in MQTT and wondering whether it’s a bug or I’m missing something. I have configured mosquitto (HA addon) to use a certificate from let’s encrypt (not self-signed), and I can connect to it from various places using the following ca file: https://letsencrypt.org/certs/isrgrootx1.pem

Eg

mosquitto_pub --cafile isrgrootx1.pem <host> ...

But it doesn’t work in nuki hub. In fact, I can’t even upload this file since the “MQTT SSL CA Certificate” field is 1500 chars max, while the cert (in PEM format) is larger. Should I use some other format?