The extension will abort on some non-recoverable errors, but can retry after other errors.
Next time this happens, please hover on the red "x" and report the error message.

Edit: I see. I have also seen this error several times, but has not found the reason yet. I'll look into it. Tracking at: https://github.com/jingyu9575/multithreaded-download-manager/issues/30

Edit (2018-06-26): A possible reason is found. Now the extension will retry, up to the "maximum retries" option. See https://github.com/jingyu9575/multithreaded-download-manager/issues/30#issuecomment-400112952 for details.