When I try to upload files using Drag and Drop I sometimes (but not always) get error 404 from EPiFileUpload. The error presents itself in certain folders but I can't see any pattern or explanation of which folders the error will appear in or why. Uploading files to these folders via the usual file upload does work. I'm not sure what could be wrong here.
We're using version 2.3 of EPiServer Client Components.
Could you enable logging in order to get more information about what's happening. Try setting the logger level to 'Debug' for the EPiServer.Web.Hosting namespace.