ERROR: Ambiguous target configuration

Hello,
I installed btrbk on a third server (two others are working seamlessly) and get the error message in subject when (dry)running.

Here below my `btrbk.conf` file:

```
#
# Example btrbk configuration file
#
#
# Please refer to the btrbk.conf(5) man-page for a complete
# description of all configuration options.
#
# Note that the options can be overridden per volume/subvolume/target
# in the corresponding sections.
#

# Enable transaction log
transaction_log /var/log/btrbk.log

# Enable stream buffer. Adding a buffer between the sending and
# receiving side is generally a good idea.
# NOTE: If enabled, make sure the "mbuffer" package is installed on
# the target host!
stream_buffer 512m

# Directory in which the btrfs snapshots are created. Relative to
# of the volume section.
# If not set, the snapshots are created in .
#
# If you want to set a custom name for the snapshot (and backups),
# use the "snapshot_name" option within the subvolume section.
#
# NOTE: btrbk does not autmatically create this directory, and the
# snapshot creation will fail if it is not present.
#
snapshot_dir _btrbk_snap

# Always create snapshots. Set this to "ondemand" to only create
# snapshots if the target volume is reachable. Set this to "no" if
# snapshot creation is done by another instance of btrbk.
#snapshot_create always

# Perform incremental backups (set to "strict" if you want to prevent
# creation of non-incremental backups if no parent is found).
#incremental yes

# Specify after what time (in full hours after midnight) backups/
# snapshots are considered as a daily backup/snapshot
preserve_hour_of_day 7

# Specify on which day of week weekly/monthly backups are to be
# preserved.
preserve_day_of_week saturday

# Preserve all snapshots for a minimum period of time.
#snapshot_preserve_min 1d

# Retention policy for the source snapshots.
#snapshot_preserve h d w m y

# Preserve all backup targets for a minimum period of time.
#target_preserve_min no

# Retention policy for backup targets:
#target_preserve h d w m y

# Retention policy for archives ("btrbk archive" command):
#archive_preserve_min no
#archive_preserve h d w m y

# Specify SSH private key for "ssh://" volumes / targets:
ssh_identity /root/.ssh/id_servername
ssh_user root
#ssh_port default
#ssh_compression no
#ssh_cipher_spec default

# Enable compression for remote btrfs send/receive operations:
#stream_compress no
#stream_compress_level default
#stream_compress_threads default

# Enable lock file support: Ensures that only one instance of btrbk
# can be run at a time.
lockfile /var/lock/btrbk.lock

# Don't wait for transaction commit on deletion. Set this to "after"
# or "each" to make sure the deletion of subvolumes is committed to
# disk when btrbk terminates.
#btrfs_commit_delete no

timestamp_format long

#
# Volume section: "volume "
#
# Directory of a btrfs volume (or subvolume)
# containing the subvolume to be backuped
# (usually the mount-point of a btrfs filesystem
# mounted with subvolid=5 option)
#
# Subvolume section: "subvolume
#
# Subvolume to be backuped, relative to
# in volume section.
#
# Target section: "target "
#
# Backup type, currently only "send-receive".
# Directory of a btrfs volume (or subvolume)
# receiving the backups.
#
# NOTE: The parser does not care about indentation, this is only for
# human readability. The options always apply to the last section
# encountered, overriding the corresponding option of the upper
# section. This means that the global options must be set before any
# "volume" section.
#

#
# Example configuration:
#
snapshot_preserve_min 7d
snapshot_preserve 14d 16w 12m

target_preserve_min no
target_preserve 30d 24w *m

archive_preserve_min latest
archive_preserve 12m 10y

# Backup to external disk mounted on /mnt/btr_backup
#volume /mnt/btr_pool
# no action if external disk is not attached
# snapshot_create ondemand

# subvolume root_gentoo
# target send-receive /mnt/btr_backup/_btrbk

# subvolume kvm
# use different retention policy for kvm backups
# target_preserve 7d 4w
# target send-receive /mnt/btr_backup/_btrbk

# Backup to external disk as well as some remote host
volume /mnt/btrfs_ssd
subvolume @
# always create snapshot, even if targets are unreachable
snapshot_create always
#target send-receive /mnt/btr_backup/_btrbk
snapshot_name system
target ssh://fileserver_ip/backups/servername

# Backup from remote host, with different naming
#volume ssh://my-remote-host.com/mnt/btr_pool
# subvolume data_0
# snapshot_dir snapshots/btrbk
# snapshot_name data_main
# target send-receive /mnt/btr_backup/_btrbk/my-remote-host.com

# Resume backups from remote host which runs its own btrbk instance
# creating snapshots for "home" in "/mnt/btr_pool/btrbk_snapshots".
#volume ssh://my-remote-host.com/mnt/btr_pool
# subvolume home
# snapshot_dir btrbk_snapshots
# snapshot_preserve_min all
# snapshot_create no
# target send-receive /mnt/btr_backup/_btrbk/my-remote-host.com
```

In the file server, I created a folder at `/backups/servername` to receive backups.
This is my `/etc/fstab` file of host:

```
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
#
UUID="xxxxxxxx" / btrfs compress=zstd,defaults,ssd,noatime,space_cache,subvol=@ 0 0
UUID="xxxxxxxx" /home btrfs compress=zstd,defaults,ssd,noatime,space_cache,subvol=@home 0 0
/dev/sda1 /boot/efi vfat umask=0077 0 1
UUID="xxxxxxxx" /mnt/btrfs_ssd btrfs compress=zstd,defaults,ssd,space_cache,noatime,subvolid=5 0 0
```

SSH id file has been stored in the fileserver. I can ssh it from the host where btrbk is running.
`_btrbk_snap` directory has been created at `/mnt/btrfs_ssd`:

```
name@hostname:$ ls -l /mnt/btrfs_ssd
total 0
drwxr-xr-x 1 root root 200 Oct 2 19:16 @
drwxr-xr-x 1 root root 120 Oct 2 20:29 _btrbk_snap
drwxr-xr-x 1 root root 0 Oct 1 21:23 cdrom
drwxr-xr-x 1 root root 18 Oct 1 21:44 @home
drwxr-xr-x 1 root root 116 Feb 1 2021 run
```
I could not find any notes about the error.
How could I investigate and fix?
Thank you.

Details:
Ubuntu Server 20.4.3.
BTRBK installed through `apt-get install btrbk` command.

Start by reproducing the dry run with the supplied btrbk.conf and the shown /mnt/btrfs_ssd layout, then inspect how the volume, subvolume, and target declarations are interpreted. Done means the configuration's ambiguous-target cause is identified and the dry run completes without the reported error.

Written by the indexing model from the issue text.