Hard to tell without a log file.

But as a wild guess, Change 1647 can be the culprit:
https://winscp.net/eng/docs/incompatible_changes#older_automation
https://winscp.net/tracker/1647

Try to replace scp://xxx@xxxxx with xxx@xxxxx

same scripts, I've upgraded to 5.15, it did not work. It was working for 5.13.8
I just downgraded to 5.13.8, it is working again.

OS:win 10
cmd line script:
"C:\Program Files (x86)\WinSCP\winscp.exe" scp://xxx@xxxxx /command "lcd C:\inetpub\webportal" "cd /mnt/c" "get xxx.txt" "exit"