FTP backup fails: Cannot create a file when that file already exists

Symptoms

FTP backup creation failed with the following error:

PLESK_ERROR: Error: Unable to export backup: Transport error: Unable to create the directory 'ftp://ftp.domain.tld/Backup/example.com/': Curl error: (21) Quote command returned error: Last FTP request: MKD example.com Last FTP response: 550 Cannot create a file when that file already exists.

OR

PLESK_ERROR: Unable to create remote backup. Error: Unable to export backup: Transport error: Unable to create the directory 'ftp://203.0.113.2/backup': Curl error: (21) Quote command returned error: Last FTP request: MKD backup Last FTP response: 550 Directory already exists

OR

PLESK_ERROR: Unable to create remote backup. Error: Unable to export backup: Transport error: Unable to create the directory 'ftp://203.0.113.2/backup': Curl error: (21) Quote command returned error: Last FTP request: MKD backup Last FTP response: 550 Can't create directory: File exists
Use passive mode option is enabled for FTP server in Tools & Settings > Backup Manager > Remote Storage Settings

Plesk product bug with id #PPPM-11295 which will be fixed in future product updates.

Until a fix became available, as a workaround, apply the Custom fix using the following steps:

For Linux

Connect to the server via SSH/RDP.
Create a backup for cuBackupAgent.php file:

# cp -ap /usr/local/psa/admin/plib/api-common/cuBackupAgent.php{,.bkp}
Download the custom cuBackupAgent.php file:

# wget https://plesk.zendesk.com/hc/article_attachments/360006728119/cuBackupAgent.zip
Unzip it and replace the original file

# unzip ./cuBackupAgent.zip
# mv ./cuBackupAgent.php /usr/local/psa/admin/plib/api-common/cuBackupAgent.php

For Windows

Connect to the server via RDP
Download the file cuBackupAgent.zip
Unzip the content of the file cuBackupAgent.zip
Back up the following file %plesk_dir%\admin\plib\api-common\cuBackupAgent.php
Place the new cuBackupAgent.php extracted in Step 3 to the %plesk_dir%\admin\plib\api-common\ folder.