ftp timeout

I am using SmartFTP to upload files to my hosting service
(webhost4life). Unfortunately, I always get a timeout with the error
"Transfer Timeout (40s). Closing data connection. Active Help:
http://www.smartftp.com/support/kb/index.php/74". The cited website
says "An established connection was aborted by the software in your
host machine. Software caused connection abort". After this the hosting
server has the uploaded file but the size is always 0 bytes.

I am using Windows Server 2003 with the firewall disabled. The machine
connects to the internet through a switch and Belkin router whose
firewall is disabled. The IP address was obtained through DHCP. The
same problem happens most of the time (not in some lucky instances)
with Windows Server 2000 and static IP address. SmartFTP is set to
passive mode and binary (same result with ASCII). The problem does not
only occur with webhost4life but other ftp sites as well. The router
has no Virtual servers defined (Firewall->virtual servers). Opening
port 21 does not help (which makes sense as this only would make my ftp
accessible from outside).

Any help would be highly appreciated.

I should add here that it works just fine with the same computer but
from a different location (work). Therefore, most likely it is some
settings in the router.