Issue with sending incremental backups

I am setting up btrbk. The first backup went fine. However, subsequent backups error out when sending the snapshot:

```
Checking for missing backups of "/var/lib/_btrbk_snap/home.*" in: phlogiston:/volume1/TARDIS/tumbolia/
Creating incremental backup...
[send/receive] target: phlogiston:/volume1/TARDIS/tumbolia/home.20250507T1738
[send/receive] source: /var/lib/_btrbk_snap/home.20250507T1738
[send/receive] parent: /var/lib/_btrbk_snap/home.20250507T1201
[send/receive] checking target metadata: phlogiston:/volume1/TARDIS/tumbolia/home.20250507T1738
ERROR: Failed to send/receive subvolume: /var/lib/_btrbk_snap/home.20250507T1738 [/var/lib/_btrbk_snap/home.20250507T1201] -> phlogiston:/volume1/TARDIS/tumbolia/home.20250507T1738
ERROR: ... failed to check target subvolume: phlogiston:/volume1/TARDIS/tumbolia/home.20250507T1738
ERROR: ... Command execution failed (exitcode=1)
ERROR: ... sh: ssh -i '/home/mario/.ssh/id_rsa' mario@phlogiston.tegu 'sudo -n btrfs subvolume show '\''/volume1/TARDIS/tumbolia/home.20250507T1738'\'''
ERROR: ... cannot find real path for '/volume1/TARDIS/tumbolia/home.20250507T1738': No such file or directory
ERROR: Error while resuming backups, aborting
```

`...T1201` is the full snapshot, `...T1738` is the incremental one. It would seem that it's checking whether the incremental snapshot exists on the remote, but instead of sending it over if it doesn't it just errors out. I'm probably doing something wrong.

Here's my `btrbk.conf` (comments removed):

```
transaction_log /var/log/btrbk.log
ssh_identity /home/mario/.ssh/id_rsa
ssh_user mario

backend_local_user btrfs-progs-sudo
backend_remote btrfs-progs-sudo

stream_buffer 256m

snapshot_dir /var/lib/_btrbk_snap

snapshot_create always

incremental yes

preserve_hour_of_day 1

preserve_day_of_week monday

stream_compress zstd
stream_compress_level 10
stream_compress_threads 4

lockfile /var/lock/btrbk.lock

snapshot_preserve_min 2d
snapshot_preserve 14d 10w *m

target_preserve_min 15d
target_preserve 20d 10w *m

target ssh://phlogiston/volume1/TARDIS/tumbolia
subvolume /sysroot
subvolume /var
subvolume /home

```

One possible issue is that the remote is a Synology NAS which is running btrfs, but uses btrfs-progs v4.0.