the ASPxUploadProgressHttpHandler for a ASPxFileManager is throwing errors suggesting it cannot find files which end in .aspx which don't even exist

This needs to be triggered by something in the Application_Start method of Global.asax.cs:

routes.Ignore("{*ashx}", new { ashx = @".*\.ashx(/.*)?" });

 
 

Addendum 3/22/2016: You may also need to exclude .js and .css files in the same manner. That said, I now suspect that this problem I'm fighting is particular to the home-rolled routing of the app in front of me.