I have trouble uploading files with SWFUpload when target servlet is listening on HTTPS. On HTTP it works fine.
The issue is only when using FireFox or Opera. On IE it works fine - not tried other browsers.
EDIT: Error message as provided from SWFUpload when in debug mode:
SWF DEBUG: Event: uploadProgress (OPEN): File ID: SWFUpload_0_0 SWF DEBUG: Event: uploadError : IO Error : File ID: SWFUpload_0_0. IO Error: Error #2038 SWF DEBUG: Event: uploadComplete : Upload cycle complete.
Possibly a certificate related issue.
Check if there are any certificate related warnings when you open the target site in those browsers.
Open the certificate and check for warnings in there as well.