Stats-writer permission denied

I've got a new Dovecot error (well, new to me, at least)!

I ran tail /var/log/mail.err and got the following:

net_connect_unix(/var/run/dovecot/stats-writer) failed: Permission denied

I checked the file, and it's owned by www-data and the www-data group, with read and write permissions for owner and group... just like I have specified in /etc/dovecot/conf.d/10-master.conf. I can't understand why permission is denied when the file permissions appear to be correct. Can anyone shed any light on this for me?

Ubuntu 20.04 Dovecot 2.3.7.2

Ken