First, it may help to review how our Remotely Hosted Downloads feature works -- see the "Here's how it works" section here:
When you get a notification that the remote download link is not accessible and the buyer was served the file from our cache, the only real problem there is that we could not verify our cached copy was still identical to your remote file -- i.e., we could not tell whether your remote file may have been updated after we'd cached a copy from a prior download, so we went ahead and served the download from that cached copy as usual, just as if we'd confirmed your remote file hadn't changed.
If you're getting this notification every time for a given file, there may be something incorrect about the Remote Product File URL you'd specified for that product; we'd be glad to take a closer look if you could tell us which product is affected, and feel free to email us if you'd rather not discuss details here on the forum:
If you're getting this notification only sometimes for a given file, while at other times the same file is delivered without notification, there's probably just some sort of sporadic networking glitch somewhere along the path across the Internet between your server and our Tucson datacenter, or possibly sporadic performance issues with your remote server, neither of which is a problem for us as long as we have a cached copy of your file to deliver to buyers. If you're sure you haven't updated your remote file since the last time that file was purchased and downloaded successfully, and if buyers aren't contacting you to say their file had a problem, you can just disregard these notifications.