A Core FTP server returns "permission denied" opening a text file from Win10 but not from Win2012.

The same Core FTP server returns the text.txt file open using WinSCP on Win 2012 server logging in with the same WinSCP version and the same user and password. I include the full debug from both logons and download attempts. The Win10 open fails while the Win2012 open succeeds.

Additionally, the Core FTP server rejects the keyfile.

It seems that you have edited the logs a lot, hiding lot of information that could be useful to help you (on the other hand, you didn't edit out your password).
Please post as complete logs as possible (you can post them as a private attachment). Normal logging level should be enough for a start (and easier to read).

One thing I could identify from the edited logs, is that on Windows 10, you are using key from Pageant, what you are not on Windows 2012. Make sure you use the same authentication in both cases.

Logs have been updated as requested.

Added server log

In Windows 10 you have /.. set as an initial remote directory, what is rather strange. Try to clear it.
https://winscp.net/eng/docs/ui_login_directories
If that does not help, please post new logs, on Normal level.

That did it. I do not know how that got there.