Excuse me, I set Let’s Encrypt to be DISABLE in the setting of Mattermost(config.json). I do not use Mattermost’s Let’s Encrypt.
I must somehow let Let’s Encrypt access the token file of .well-known/acme-challenge.
I tried specifying the .well-known folder in the LetsEncryptCertificateCacheFile entry in config.json, but it seems Let’s Encrypt can not access the token file as well.