pg_basebackup fails with "COPY stream ended"

Hi,

I am running "pg_basebackup -h <host> -U postgres -D <dir> -X stream". It
fails with either of the following two error messages:

ERROR: Backup failed copying files.
DETAILS: data transfer failure on directory
'/mnt/data/barman/base/20210615T212304/data'
pg_basebackup error:
pg_basebackup: initiating base backup, waiting for checkpoint to complete
pg_basebackup: checkpoint completed
WARNING: terminating connection because of crash of another server process
DETAIL: The postmaster has commanded this server process to roll back the
current transaction and exit, because another server process exited
abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the database and
repeat your command.
pg_basebackup: error: could not read COPY data: SSL SYSCALL error: EOF
detected
pg_basebackup: removing contents of data directory
"/mnt/data/barman/base/20210615T212304/data"

OR,

ERROR: Backup failed copying files.
DETAILS: data transfer failure on directory
'/mnt/data/barman/base/20210615T212849/data'
pg_basebackup error:
pg_basebackup: initiating base backup, waiting for checkpoint to complete
pg_basebackup: checkpoint completed
pg_basebackup: error: COPY stream ended before last file was finished
pg_basebackup: removing contents of data directory
"/mnt/data/barman/base/20210615T212849/data"

I am running Postgres 12.7 on Ubuntu 20.04. Can anyone please help me
figure out what's going wrong?