This behavior is due to a bug in the ISAPI filter in the 4.0.8 product version of FileUp. The ISAPI filter is a dll which comes with FileUp product and is mainly used to enable large uploads in ASP.NET. To be certain that the behavior you are experiencing is a result of this faulty dll, you will need to confirm the dll version of the ISAPI filter.

How do I know if I'm running the problematic dll?
Locate the ISAPI filter dll on your webserver and check the version number. This dll usually installs to: "C:\Program Files\Software Artisans\FileUp\FileUpIsapi.dll". Right-click on the dll and choose "properties" from the context menu. Select the "version" tab. The faulty dll has a version number of "4.0.4.411". This is the version of the ISAPI dll that is included in version 4.0.8 of FileUp. If you are running this version, you simply need to upgrade to version 4.0.9 or higher of FileUp to get the corrected ISAPI dll.

My ISAPI dll (FileUpIsapi.dll) version number is not 4.0.4.411. What else could be causing this behavior?
Please see the following KB article for an alternate cause: Server-side Progress Indicator Fails to Refresh