FTPS backup doesn't work with Hetzner Storage box

Look at the FTP/FTPS client code in the plugin, likely in classes handling remote storage. The error 'Couldn't access ... file does not exist' during upload suggests a path or permission issue with the storage box. Start by examining the FTP job class and the specific methods for upload and directory handling. Test with a simple FTPS connection to see if the server requires a different mode or path prefix.

Describe the bug
I am using the latest version of BackWPUp 5.7.6 on my wordpress site and I am trying to backup to Hetzner Storage box via FTPS.
I have created a subuser on the storage node, which has access to /website/mywebsite and this location is the root location for the subuser.

Backup fails with the error below.

 [INFO] BackWPup 5.7.6; A project of WP Media
[INFO] WordPress 6.9.9 on http://www.domain.al/
[INFO] Log Level: Normal
[INFO] BackWPup job: Full
[INFO] Logfile is: backwpup_log_63db0f_2026-09-23_06-00-09.html
[INFO] Backup file is located at: /home/user/public_html/wp-content/uploads/.../2026-09-23_06-00-09_KFVE5G3S02_FILE-DBDUMP-WPPLUGIN.tar.gz
[September 23, 2026 6:00 am] 1. Try to backup database …
[September 23, 2026 6:00 am] Connected to database user_wpress on localhost
[September 23, 2026 6:00 am] Added database dump "user_wpress.sql" with 25.55 MB to backup file list
[September 23, 2026 6:00 am] Database backup done!
[September 23, 2026 6:00 am] 1. Trying to make a list of folders to back up …
[September 23, 2026 6:00 am] Added "wp-config.php" to backup file list
[September 23, 2026 6:00 am] 2265 folders to backup.
[September 23, 2026 6:00 am] 1. Trying to generate a file with installed plugin names …
[September 23, 2026 6:00 am] Added plugin list file "website.pluginlist.2026-09-23.txt" with 2.28 KB to backup file list.
[September 23, 2026 6:00 am] 1. Trying to generate a manifest file …
[September 23, 2026 6:00 am] Added manifest.json file with 3.26 KB to backup file list.
[September 23, 2026 6:00 am] 1. Trying to create backup archive …
[September 23, 2026 6:00 am] Compressing files as TarGz. Please be patient, this may take a moment.
[September 23, 2026 6:05 am] Backup archive created.
[September 23, 2026 6:05 am] Archive size is 1.10 GB.
[September 23, 2026 6:05 am] 21428 Files with 1.30 GB in Archive.
[September 23, 2026 6:05 am] 1. Try to send backup file to an FTP server …
[September 23, 2026 6:05 am] Connected via explicit SSL-FTP to server: username-sub3.your-storagebox.de:21
[September 23, 2026 6:05 am] FTP client command: HELP
[September 23, 2026 6:05 am] FTP server response: 530 Please login with USER and PASS
[September 23, 2026 6:05 am] FTP client command: USER username
[September 23, 2026 6:05 am] FTP server response: User username logged in.
[September 23, 2026 6:05 am] FTP client command: PASV
[September 23, 2026 6:05 am] FTP server reply: Entering passive mode
[September 23, 2026 6:05 am] FTP current folder is: /backwpup/
[September 23, 2026 6:05 am] Starting upload to FTP  …
[September 23, 2026 6:05 am] WARNING: Looks like the FTP Server do not support upload files in chunks. Try to upload it at once without restarts.
[September 23, 2026 6:05 am] ERROR: FTP server error: FTP server response: Couldn't access /backwpup/2026-09-23_06-00-09_KFVE5G3S02_FILE-DBDUMP-WPPLUGIN.tar.gz: file does not exist
[September 23, 2026 6:05 am] 2. Try to send backup file to an FTP server …
[September 23, 2026 6:05 am] Connected via explicit SSL-FTP to server: username-sub3.your-storagebox.de:21
[September 23, 2026 6:05 am] FTP client command: HELP
[September 23, 2026 6:05 am] FTP server response: 530 Please login with USER and PASS
[September 23, 2026 6:05 am] FTP client command: USER username
[September 23, 2026 6:05 am] FTP server response: User username-sub3 logged in.
[September 23, 2026 6:05 am] FTP client command: PASV
[September 23, 2026 6:05 am] FTP server reply: Entering passive mode
[September 23, 2026 6:05 am] FTP current folder is: /backwpup/
[September 23, 2026 6:05 am] Starting upload to FTP  …
[September 23, 2026 6:05 am] WARNING: Looks like the FTP Server do not support upload files in chunks. Try to upload it at once without restarts.
[September 23, 2026 6:05 am] ERROR: FTP server error: FTP server response: Couldn't access /backwpup/2026-09-23_06-00-09_KFVE5G3S02_FILE-DBDUMP-WPPLUGIN.tar.gz: file does not exist
[September 23, 2026 6:05 am] 3. Try to send backup file to an FTP server …
[September 23, 2026 6:05 am] Connected via explicit SSL-FTP to server: username-sub3.your-storagebox.de:21
[September 23, 2026 6:05 am] FTP client command: HELP
[September 23, 2026 6:05 am] FTP server response: 530 Please login with USER and PASS
[September 23, 2026 6:05 am] FTP client command: USER username-sub3
[September 23, 2026 6:05 am] FTP server response: User username-sub3 logged in.
[September 23, 2026 6:05 am] FTP client command: PASV
[September 23, 2026 6:05 am] FTP server reply: Entering passive mode
[September 23, 2026 6:05 am] FTP current folder is: /backwpup/
[September 23, 2026 6:05 am] Starting upload to FTP  …
[September 23, 2026 6:05 am] WARNING: Looks like the FTP Server do not support upload files in chunks. Try to upload it at once without restarts.
[September 23, 2026 6:05 am] ERROR: FTP server error: FTP server response: Couldn't access /backwpup/2026-09-23_06-00-09_KFVE5G3S02_FILE-DBDUMP-WPPLUGIN.tar.gz: file does not exist
[September 23, 2026 6:05 am] ERROR: Step aborted: too many attempts!
[September 23, 2026 6:05 am] ERROR: Job has ended with errors in 319 seconds. You must resolve the errors for correct execution.