Error when transferring .dss files

[NT]

We are using Core FTP Lite. We have 2 doctors whose files are in .dss format. The files for one doctor will transfer fine, but when trying to transfer files for the second doctor only 1 file will transfer and then will get a Windows error message that an error has occurred and needs to shut down. This happens whether we try to transfer this doctor's files first or second. We are using XP Professional.