Hi Robert,
This error seems very similar to this one:
Error on UploadHowever, that error was fixed back in v3.2.2. You say you are using the latest version so this is strange.
If it's the same error you should experience it in Flash upload mode when uploading more than one file at once. This may explain why you didn't experience it all the time.
I recommend you to check versions of dlls in FileVista\bin folder. Please make sure especially GleamTech.Web.FileTransfer.dll is from the latest package.
Let me know.