Permission issues with dovecot stats-writer

Env: Debian 10

Today I encountered an error when trying to receive new messages from my mailbox.
Then I checked /var/log/mail.log and found this:

Jul 11 10:54:00 mail postfix/pipe[27149]: 0A3FBDBE26: to=<xxx@snorl.ax>, orig_to=<sim@snorl.ax>, relay=spamassassin, delay=1.4, delays=0.45/0.02/0/0.98, dsn=2.0.0, status=sent (delivered via spamassassin service (lda(xxx@snorl.ax,)Error: net_connect_unix(/var/run/dovecot/stats-writer) failed: Permission denied))

I googled and found this solution1 work well. Just append the following settings to /etc/dovecot/dovecot.conf:

service stats {
    unix_listener stats-reader {
        user = vmail
        group = vmail
        mode = 0660
    }

    unix_listener stats-writer {
        user = vmail
        group = vmail
        mode = 0660
    }
}

Then restart dovecot:

sim@mail:~$ sudo systemctl restart dovecot