[DevExpress Support Team: CLONED FROM B135843: ASPxUploadControl - ASPxUploadProgressHandlerPage.ashx does not exist]
I found some issues with this.
It came down to a missing folder, permissions on that folder and IIS being set the to Integrated vs. Classic.
Missing Folder was /App_Data/UploadTemp
Set Permissions for NETWORK SERVICE on that folder and it worked.
Did I miss this in the documentation someplace or was there another reason it was missing?
Can I change that path or use a different path if that one doesn't work for me?
I hope this helps.