Comment # 1 on bug 954959 from Christian Boltz

I'd guess you need to add

  owner /tmp/dovecot.lda.* rw,

to /etc/apparmor.d/usr.lib.dovecot.dovecot-lda and reload AppArmor after that
("rcapparmor reload").

If this doesn't help, please remove the added line again, switch the profile
into complain mode, trigger the event again and attach the log (as described in
bug 954958).