@Joulinar: I don’t understand why it should be better to restore data from a Linux system than from a Windows system - the data itself is not changed. And the old, unknown configured system should not be restored either (explicit statement of the owner: “Only save my data and setup a fresh new system").
@StephanStS: Your tip provided exactly the quick solution I was looking for. Thanks for the hint. 
My personal solution for this issue:
- The Linux SSD was connected to a Windows PC via USB adapter.
- The free application “Diskinternals Linux Reader” was used to find the old data in the Linux file system and back it up locally.
- Using the free application “WinSCP”, the recovered data was copied to the new target system via SFTP protocol (beforehand, it was ensured that “OpenSSH Server” (and NOT “Dropbear”) was selected under “dietpi-software” and there under “SSH Server” - otherwise SFTP is not possible).
- Then a new scan had to be triggered on the target system:
- Change to the Nextcloud directory:
~ cd /var/www/nextcloud
- Initiate the new Netcloud file scan:
~ sudo -u www-data php occ files:scan -all
The users new fresh system (a refurbished DELL Wyse 5070) is now up and runinng, the owner is happy to have his data rescued, and I have another IT-Job to do regularly (doing Backups).
Cheers,
Lelo