2.2.18: Mailbox INBOX sync: mailbox_delete failed: INBOX can't be deleted.

[Dale Gallagher]

Hello

I have 2.2.18 built with:

./configure \
    --prefix="$prefix" \
    --bindir="$prefix/bin" \
    --sbindir="$prefix/bin" \
    --sysconfdir="/etc" \
    --with-storages="maildir,imapc,pop3c" \
    --without-vpopmail \
    --without-ldap

I'm attempting to sync (backup) from another Dovecot server running version
2.0.21:

Both servers don't have no explicit namespaces configured and both are
using the Maildir storage plugin.

doveadm -Dv -o imapc_user=remote_user -o imapc_host=source.host -o
imapc_port=993 -o imapc_ssl=imaps -o imapc_password=remote_pass -o
imapc_features="rfc822.size fetch-headers" backup -R -u local_user imapc:

All seems to go well, showing local and remote mailbox syncing without
errors, then finally:

dsync(mcg): Debug: brain M: Deleting mailbox 'INBOX' (GUID
59023c2a911dc255842b0000d09efc50): UIDVALIDITY changed (1355416734 ->
1438784913)
dsync(mcg): Error: Mailbox INBOX sync: mailbox_delete failed: INBOX can't
be deleted.

No mail is synced to the local server (2.2.18) after this error halts
execution.

What should I be looking for, in terms of possible causes of this error?

I've tried syncing on a completely fresh mailbox, which hasn't been
accessed by a mail client and the same error occurs.

I've seen similar reports of this error before, without clear solutions /
suggestions.

Help would be greatly appreciated.

Thanks