I can't unfortunately tell what goes wrong with the download. What I can tell you, though - the code that handles downloads and progress bar is indeed pretty badly implemented and it does block the main event loop when downloading. I actually have a patch prepared for that, and hopefully it will go in into some future beta release. Until then, I don't know how to fix the downloads, sorry
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Are the downloads only stuck on maps for you? If that's the case, I suppose a clunky workaround would be to manually download the maps from
http://content.faforever.com/faf/vault/maps/<mapname>.zip and unzipping them in your map folder.