A user reported this to me, and I'm seeing it too:

Hi all :)

After mail/dovecot upgrade to v2.3.7 it seems something went wrong with stat-writer...

IMAP and POP3 logs:

Jul 12 19:45:34 mail dovecot: imap-login: Error: file_ostream.net_set_tcp_nodelay((conn:unix:/var/run/dovecot/stats-writer), TRUE) failed: Invalid argument