What this error means
Resuming relies on range requests: XDM asks for “bytes from position X onward” and the server sends only that part. It’s also what makes XDM fast, because segments of the file download in parallel. Some servers, especially those generating files on the fly or streaming dynamic content, ignore range requests and always send the whole file. If such a download is interrupted, there’s no way to continue it.
What you can do
- Download it again from the start. Delete the failed entry and start the download again from the web page (or paste the link with New in the toolbar).
- Keep the connection stable until it completes: avoid sleep mode (enable Keep the computer awake in Settings → Advanced) and pausing.
- Try another mirror. Download sites often offer several servers. A different mirror may support resume and download much faster.
Why such downloads are slower
Without range support XDM can only use a single connection, so acceleration doesn't apply. That's a limitation of the server, not of XDM or your connection.