Thanks for the response - the file is there:
total 15
drwxr-xr-x 1 zimbra zimbra 4096 Mar 7 12:04 blobs
drwxr-xr-x 1 zimbra zimbra 4096 Mar 7 12:04 db
drwxr-xr-x 1 zimbra zimbra 4096 Mar 7 12:04 index
-rwxr-xr-x 1 zimbra zimbra 1618 Mar 8 01:00 ldap.xml
-rwxr-xr-x 1 zimbra zimbra 530 Mar 8 01:00 meta.xml
And it does seem to contain valid data:
<backupMetadata xmlns="urn:zimbraBackupMeta" version="7.0">
<accountBackup zimbraId="e08bc6bd-d835-4262-8a56-7365c3313789" email="zimbra@phosphor" mailboxId="36" startTime="1204956019727" endTime="12049
56020594" redoSeq="35" server="phosphor">
<volumes>
<volume id="2" path="/opt/zimbra/index/0/36/index">
</volume>
<volume id="1" path="/opt/zimbra/store/0/36/msg">
</volume>
</volumes>
</accountBackup>
</backupMetadata>
I wonder why it's not permitted to read it...