Any FTP user home directories turn to "/" instead of the one set in Plesk.
Plesk FTP site (10.10.10.10) is stoped in IIS and could not be started with the error:
This FTP site cannot be started. Another FTP site may be using the same port.
Another FTP site is using the same port.
Check in IIS if any other site is bonded to 21 port on the same IP address and delete it.