What version of SmartFTP are you running?
I've just Googled "An existing connection was forcibly closed by the remote host", & one result gave this:
http://www.smartftp.com/phpBB2/viewtopic.php?t=9038&
Someone got the same error with a recent build. If this is actually the same as your problem, it's apparently been fixed in v1.5.988.25
Other than that...no ideas, sorry.