CentOS 5 Server :: SFTP Login For "root" Stopped Working?

I've been administrating a dedicated Linux CentOS 5 (Linux 2.6.26.5-rootserver-20080917a) server for around 2 years, and although not a network or Linux expert, been learning to configure as need arises. Primarily using Plesk for day-to-day, but occasionally using Putty to SSH into server.

For all the time I've had the server, I've been connecting to my server via sFTP using "root" password. ( Although, I know this is really bad practice, I assume made safer by connecting with SSH FTP)

After spending another normal day in the office developing websites, connecting to my server as root using SFTP in Filezilla AND Dreamweaver I left for the night.

Returned next morning, after having done no manual updates or amends to my server; I could no longer SFTP into my server?

Thought it may be related to my office network, so tried it from home over the weekend, same result; can no longer connect SFTP for root?

I can connect to the server via Putty using my "root" username and password.

After spending hours looking on the internet for a solution, I'm lost for ideas as I didn't make any changes?

What happens when I open my Filezilla and try connecting as SFTP is it states:

Error:Connection timed out
Error:Could not connect to server

I checked server log /var/log/secure and it states:

Accepted password for root from UNKNOWN port 49212 ssh2
Apr 9 07:41:41 s15320264 sshd[7122]: fatal: Write failed: Connection reset by peer

Odd part is, it's worked fine for weeks, months without ever failing to connect?

Also, notice that Putty connection seems to take much longer to authenticate root user than it used to?

Checked via Plesk Health Monitoring and all CPU, Memory and Disk Levels are well below any alarm levels.

I have run all Plesk updates to 10.2.0 in the hope that it resolved it, but to no avail.