This error may be caused by multiple reasons, such as:
1. AspUpload is not installed on the server
Make sure the AspUpload component is installed on the server. JUpload requires
AspUpload to capture uploaded files.
2. Upload script throws an error
The upload script pointed to by the UploadURL parameter
may not be working properly. There can be many reasons
why an upload script would fail, such as failure to create the Upload object,
a permission problem when saving an uploaded file, invalid syntax, etc.
You must make sure that your upload script is functional. The best way to do it is to
create a simple HTML file with an upload form
and point this form's ACTION attribute to the same URL as your UploadURL parameter. Then
try to upload a small file using this form. If your upload script has a problem,
you will be able to see it immediately. Your form may look like this:
<FORM ACTION="http://server/path/script.asp" METHOD="POST" ENCTYPE="multipart/form-data">
<INPUT TYPE="FILE" NAME="FILE1">
3. Server-side upload script times out
When uploading large files, the upload process may take several minutes
and as a result the server-side upload script may time out.
You should increase the Script Timeout value
for your virtual directory from MMC (IIS4) or Internet Services Manager (IIS5)
under Application Configuration/App Options.