Dovecot 2.4.2 auth unix socket rejects Postfix 3.10.6 handshake with EACCES

Environment: Ubuntu 26.04 (resolute), kernel 7.0.0-30-generic, Postfix 3.10.6-4ubuntu2.1, Dovecot 2.4.2+dfsg1-3ubuntu2.1.

Problem: Postfix smtpd (submission) connects to /var/spool/postfix/private/auth but write(VERSION...CPID...) returns EACCES. Log: "warning: SASL: Couldn't send handshake: Permission denied", then "fatal: no SASL authentication mechanisms".

Socket permissions are correct (postfix:postfix 0660). Writing to the same socket as user postfix from python works. AppArmor in complain mode and removed profile - no change. Running postfix master manually (not via systemd) still fails. Only real smtpd process fails, python with same uid/gid succeeds.

Workaround: inet listener on 127.0.0.1:3456 for auth works, AUTH succeeds.