I think replays (at least MM ones) get deleted after 3 weeks. Not 100% sure on this though.
I think this may answer your problems; http://www.reddit.com/r/Dota2Trade/comments/1996vk/psa_how_to_solve_replay_download_error/
If all else fails, you can just check it out on youtube..
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It happens to me that there isn´t a "Download Replay" Button anymore but only the "Watch Replay" which was never working for me.
Is is anyhow hidden somewhere or can I download a replay somewhere else?
Help would be much appreciated. Thanks