Make sure you've got enough disk space to save your files. Check the SessionTimeout and ScriptTimeout properties as mentioned in the'Script Settings' portion of the documentation. This means that your system drive has to have enough space. If you have five simultaneous 100 MB uploads you'll need minimum 500 MB free disk space. Your client browser has to have disk space free.
Many browsers save the encoded form on the system disk before sending it through to the web server.
Due to bugs in IE this cache can be as much as twice as large as the uploaded files. If your client does not have sufficient space free the results can be unpredictable. There is a theoretical limit to the number of large uploads available. This is a limit that is only likely to come into play for large numbers of very large uploads. If this doesn't help please mail us with an exact description of the problem and the system on which you're running.