[Dovecot] "userdb didn't return a home directory" with v1.1.7

[Ivars Strazdiņš]

Hi all,
I am running Dovecot v1.1.7 in a relatively quiet and calm environment. 
Suddenly overnight cron job started throwing out errors like:
/usr/sbin/dovecot --exec-mail ext /usr/lib/dovecot/expire-tool

Error: userdb(user1 at mail.example) didn't return a home directory
Error: userdb(user2 at mail.example) didn't return a home directory

Indeed, running this command produces:
-------------------------
$ sudo /usr/sbin/dovecot --exec-mail ext /usr/lib/dovecot/expire-tool --test
Error: userdb(user1 at mail.example) didn't return a home directory
Info: User lookup failed: user1 at mail.example
Error: userdb(user2 at mail.example) didn't return a home directory
Info: User lookup failed: user2 at mail.example
-------------------------

Yet these mailboxes are valid, corresponding users can login, send and 
receive mail. Other mailboxes are OK, too.
Where to to start looking? I found that similar error message with 
v1.1.6 is fixed in v1.1.7, but I am running v.1.1.7 already.
Please see dovecot configuration attached.
Thank you for your time,
Ivars
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