Urbackup unable to use btrfs partition

Question

I'm trying to get urbackup 2.4.9.0 to work on Ubuntu 19.10 x86. I have a 2T usb drive partitioned as btrfs, but it's not recognized as btrfs by urbackup_snapshot_helper.

/etc/urbackup/backupfolder contains:
/media/BACKUP/urbackup

The test program suggested in https://www.urbackup.org/administration_manual.html#x1-10600011.6.3 says that /media/BACKUP/urbackup/testA5... is not btrfs. I don't know where the ...TestA5... came from.

% urbackup_snapshot_helper test
Testing for btrfs...
ERROR: not a btrfs filesystem: /media/BACKUP/urbackup/testA54hj5luZtlorr494
TEST FAILED: Creating test btrfs subvolume failed
Testing for zfs...
# parted -l /dev/sda1
Model: Seagate Ultra Slim MT (scsi)
Disk /dev/sda: 2000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos
Disk Flags: 

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  2000GB  2000GB  primary  btrfs        boot

Here are the contents of the usb drive:

% ls -l /media/BACKUP/
/media/BACKUP/:
total 4
drwxr-xr-x 5 urbackup urbackup 4096 Oct 18 17:45 urbackup/

/media/BACKUP/urbackup:
total 12
drwxr-x--- 2 urbackup urbackup 4096 Jan 21  2018 clients/
drwxr-x--- 2 urbackup urbackup 4096 Oct 16 03:26 urbackup/
drwxr-x--- 2 urbackup urbackup 4096 Oct 18 16:56 urbackup_tmp_files/

/media/BACKUP/urbackup/clients:
total 0

/media/BACKUP/urbackup/urbackup:
total 60620
-rwxr-x--- 1 urbackup urbackup 52428800 Oct 16 03:25 backup_server.db*
-rwxr-x--- 1 urbackup urbackup  9051672 Oct 16 03:25 backup_server.db-wal*
-rwxr-x--- 1 urbackup urbackup    16384 Oct 16 03:26 backup_server_files.db*
-rwxr-x--- 1 urbackup urbackup        0 Oct 16 03:26 backup_server_files.db-wal*
-rwxr-x--- 1 urbackup urbackup     8192 Oct 16 03:26 backup_server_link_journal.db*
-rwxr-x--- 1 urbackup urbackup   379072 Oct 16 03:26 backup_server_link_journal.db-wal*
-rwxr-x--- 1 urbackup urbackup    16384 Oct 16 03:26 backup_server_links.db*
-rwxr-x--- 1 urbackup urbackup        0 Oct 16 03:26 backup_server_links.db-wal*
-rwxr-x--- 1 urbackup urbackup    61440 Oct 16 03:26 backup_server_settings.db*
-rwxr-x--- 1 urbackup urbackup    86552 Oct 16 03:26 backup_server_settings.db-wal*
-rwxr-x--- 1 urbackup urbackup      391 Oct 16 03:26 server_ident_ecdsa409k1.priv*
-rwxr-x--- 1 urbackup urbackup      436 Oct 16 03:26 server_ident_ecdsa409k1.pub*
-rwxr-x--- 1 urbackup urbackup       23 Oct 16 03:26 server_ident.key*
-rwxr-x--- 1 urbackup urbackup      334 Oct 16 03:26 server_ident.priv*
-rwxr-x--- 1 urbackup urbackup      442 Oct 16 03:26 server_ident.pub*

/media/BACKUP/urbackup/urbackup_tmp_files:
total 0

Any idea of what I'm doing wrong?

---

I tried to attach the log from running "strace -f urbackup_snapshot_helper test", but I see no way to attach a file, and pasting the contents is too large for this buffer.

Suggestions?

The output of "urbackup_snapshot_helper test" is: Testing for btrfs... ERROR: not a btrfs filesystem: /media/BACKUP/urbackup/testA54hj5luZtlorr494 TEST FAILED: Creating test btrfs subvolume failed Testing for zfs... TEST FAILED: Dataset is not set via /etc/urbackup/dataset

I don't know why it says that /media/BACKUP/urbackup/testA54hj5luZtlorr494 is not a btrfs filesystem.

/media/BACKUP/urbackup IS a btrfs.