dovecot - Oct 2019 - Panic at "doveadm backup" (mailbox_symlink_exists): assertion failed: (ret > 0)

Hi.

I try to sync from a dovecot server to another server via the following command.

doveadm -D -o mail_fsync=never -o imapc_user=info at DOMAIN.COM -o
imapc_password='PASS' backup -R -u info at DOMAIN.COM imapc:

After some short runtime I get the following panic.

```
dsync(info at DOMAIN.COM): Panic: file mailbox-alias-plugin.c: line 77
(mailbox_symlink_exists): assertion failed: (ret > 0)

dsync(info at DOMAIN.COM): Error: Raw backtrace:
/usr/lib64/dovecot/libdovecot.so.0(+0xa17ce) [0x7ff2f4da77ce] ->
/usr/lib64/dovecot/libdovecot.so.0(default_fatal_handler+0x2a) [0x7ff2f4da783a]
-> /usr/lib64/dovecot/libdovecot.so.0(i_fatal+0) [0x7ff2f4d36dd7] ->
/usr/lib64/dovecot/lib20_mailbox_alias_plugin.so(+0x133e) [0x7ff2f3afa33e] ->
/usr/lib64/dovecot/lib20_mailbox_alias_plugin.so(+0x14d4) [0x7ff2f3afa4d4] ->
/usr/lib64/dovecot/lib01_acl_plugin.so(+0xf2aa) [0x7ff2f432a2aa] ->
/usr/lib64/dovecot/lib01_acl_plugin.so(+0xf908) [0x7ff2f432a908] ->
/usr/lib64/dovecot/libdovecot-storage.so.0(mailbox_list_iter_next+0x44)
[0x7ff2f50ec5e4] -> doveadm(dsync_mailbox_tree_fill+0x19b) [0x558004586e5b]
->
doveadm(dsync_brain_mailbox_trees_init+0x124) [0x55800457b204] ->
doveadm(dsync_brain_run+0x3d7) [0x558004578317] -> doveadm(+0x29b71)
[0x55800455eb71] -> doveadm(+0x2bd2c) [0x558004560d2c] ->
doveadm(+0x2c92a)
[0x55800456192a] -> doveadm(doveadm_mail_try_run+0x215) [0x558004562285]
->
doveadm(main+0x46a) [0x5580045513fa] ->
/lib64/libc.so.6(__libc_start_main+0xf5)
[0x7ff2f495a505] -> doveadm(+0x1c565) [0x558004551565]
```

There is this message in the archive wich mention that similar directory names
could be a problem.

https://dovecot.org/list/dovecot/2016-December/106528.html

I have cleanuped the similar directories but I still get the panic.

This are the main folders in the Account
```
... some custom folders
=Trash
=Sent Messages
=Junk
=Drafts
=Coaching
=Archiv
=INBOX
```

Dovecot version
# 2.2.36.4 (baf9232c1): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.24.2 (aaba65b7)
# OS: Linux 3.10.0-1062.1.2.el7.x86_64 x86_64 CentOS Linux release 7.7.1908
(Core)

Thanks for any help

Regards
Alex