Description of problem:
Once an online backup fails, the next attempts will be defective:

...
[14/Apr/2022:11:50:22.007888873 +0200] - INFO - task_backup_thread - Beginning backup of 'ldbm database'
[14/Apr/2022:11:50:22.010284307 +0200] - WARN - ldbm_back_ldbm2archive - Backend 'userRoot' is already in the middle of another task and cannot be disturbed.
[14/Apr/2022:11:50:22.012907630 +0200] - ERR - ldbm_back_ldbm2archive - Failed removing /local/backup_ds/backup-2022_04_14_11_50_21
[14/Apr/2022:11:50:22.016358296 +0200] - ERR - task_backup_thread - Backup failed (error -1)
[14/Apr/2022:11:50:24.696442611 +0200] - INFO - task_backup_thread - Beginning backup of 'ldbm database'
[14/Apr/2022:11:50:24.699312491 +0200] - WARN - ldbm_back_ldbm2archive - Backend 'userRoot' is already in the middle of another task and cannot be disturbed.
[14/Apr/2022:11:50:24.701486711 +0200] - ERR - ldbm_back_ldbm2archive - Failed removing /local/backup_ds/backup-2022_04_14_11_50_24

Version-Release number of selected component (if applicable):
$ cat /etc/redhat-release
Red Hat Enterprise Linux release 8.4 (Ootpa)
$
$ rpm -qa | grep 389-ds-base-1
389-ds-base-1.4.3.22-1.module+el8dsrv+10501+8ce33e95.x86_64
$

How reproducible:
I can reproduce the issue quite reliably.

Steps to Reproduce:
1. Start an online backup.
2. While the backup is still running, delete the backup files ( under the location specified by the "nsslapd-bakdir" parameter )
3. The current backup will fail
4. Try to run again online backups. They will fail with the following message in the errors log:
Backend 'XXX' is already in the middle of another task and cannot be disturbed.

Restarting the RHDS instance fixes the issue.
This looks quite similar to bug https://bugzilla.redhat.com/show_bug.cgi?id=1642838

Actual results:
Failing to run online backups.

Expected results:
Successful online backups.

Additional info:

$ dsconf -v -D "cn=Directory Manager" ldap://localhost:389 backup create
...
DEBUG: complete status: -1 -> Backup failed (error -1)
DEBUG: cn=backup_2022-04-14T11:51:33.785070,cn=backup,cn=tasks,cn=config getVal('nsTaskExitCode')
DEBUG: cn=backup_2022-04-14T11:51:33.785070,cn=backup,cn=tasks,cn=config getVal('nsTaskLog')
DEBUG: cn=backup_2022-04-14T11:51:33.785070,cn=backup,cn=tasks,cn=config getVal('nsTaskWarning')
DEBUG: cn=backup_2022-04-14T11:51:33.785070,cn=backup,cn=tasks,cn=config getVal('nsTaskStatus')
DEBUG: complete status: -1 -> Backup failed (error -1)
DEBUG: cn=backup_2022-04-14T11:51:33.785070,cn=backup,cn=tasks,cn=config getVal('nsTaskExitCode')
DEBUG: cn=backup_2022-04-14T11:51:33.785070,cn=backup,cn=tasks,cn=config getVal('nsTaskLog')
DEBUG: cn=backup_2022-04-14T11:51:33.785070,cn=backup,cn=tasks,cn=config getVal('nsTaskWarning')
DEBUG: cn=backup_2022-04-14T11:51:33.785070,cn=backup,cn=tasks,cn=config getVal('nsTaskStatus')
DEBUG: complete status: -1 -> Backup failed (error -1)
DEBUG: The backup create task has failed with the error code: (-1)
Traceback (most recent call last):
File "/usr/sbin/dsconf", line 134, in <module>
result = args.func(inst, None, log, args)
File "/usr/lib/python3.6/site-packages/lib389/cli_conf/backup.py", line 20, in backup_create
raise ValueError("The backup create task has failed with the error code: ({})".format(result))
ValueError: The backup create task has failed with the error code: (-1)
ERROR: Error: The backup create task has failed with the error code: (-1)
$