FTP adapter stopped polling

The project required files to be picked up by BizTalk from two locations on a UNIX box.
We chose to set up two FTP sites on the UNIX box and have BizTalk poll the two sites using the FTP adapter. This approach seemed to work fine for a long while. There was never a problem on the development environment, and for a couple of weeks everything worked fine in production as well.
Then all of the sudden the FTP receive location for either or both sites would stop polling the site. Looking in the BizTalk Administration Console would show no signs of something being wrong, the receive locations would be enabled, the eventlog would show no error and the Group Hub Page would show nothing unusual.
The log on the UNIX box would just show that at a given time one or both polling activities would stop.
Restarting the Host Instance would immediately make the receive location pick up the files that were piling up in the FTP folder, and everything would work as expected for some time again. But only for a while. After a few days the same situation would occur.

I never managed to find an actual fix for this problem, but I eventually did find out that the clients UNIX OS was not on the list of supported operating systems for the BizTalk FTP adapter (http://support.microsoft.com/kb/841478). Our client was running SCO.
There is a fix for a problem with similar symptoms concerning IBM AIX based FTP servers (http://support.microsoft.com/kb/932886), but we never tried installing that, as we chose a completely different approach: Windows Services for UNIX (SFU).