Failed to get attribute vendor/vendor.dovecot/pvt/server/sieve/default

[Bob Smith]

Any ideas what this cryptic message means ? Despite what the message says, there is no "more information" to be found in the logs on either server !

mail.debug dovecot: dsync-server(<REMOVED>): Debug: sieve: file storage: Created storage directory /my/dir/structure/.dovecot.sieve/tmp
mxd mail.debug dovecot: dsync-server(<REMOVED>): Debug: sieve: file storage: Relative path to sieve storage in active link: .dovecot.sieve/
mxd mail.debug dovecot: dsync-server(<REMOVED>): Debug: sieve: file storage: sync: Synchronization active
Error: Mailbox INBOX: Failed to get attribute vendor/vendor.dovecot/pvt/server/sieve/default: Internal error occurred. Refer to server log for more information. [2018-09-03 09:52:46]
mail.debug dovecot: dsync-server(<REMOVED>): Debug: brain S: Import INBOX: Import attribute vendor/vendor.dovecot/pvt/server/sieve/default: failed
mail.debug dovecot: dsync-server(<REMOVED>): Debug: brain S: Import INBOX: Saved UIDs:

No errors are reported by "doveadm -v backup my at user tcp:mydest:myport".

doveadm-backup returns exit code 75, something which also appears to be documented nowhere !