I recently ran into a problem where an application could not be downloaded to a client.
Upon investigation I discovered that the Source Media contained files that had a Plus (+) sign in the file name, by default IIS won’t \ can’t deal with this and you end up with a “404 File not found” error.
If you enable Double Escaping on the Distribution Points then IIS can deal with the plus sign and the file can be downloaded.
appcmd set config /section:requestfiltering /allowdoubleescaping:true
Be aware that this does reduce the security of your IIS Server, and your distribution point may be vulnerable to malicious URL’s. Since this is only a Distribution Point, this should be a minor issue.