Just in case it helps, I am getting this for a different game, here is the log:
[2017-10-27 @ 17:19:53.545] [reactors/tasks] Should download 182904, has dl key ? false
[2017-10-27 @ 17:19:53.581] [download-watcher] 85fc615b-abdc-4b23-a29b-1c0dab929912 is the new active download
[2017-10-27 @ 17:19:53.581] [download-watcher] Starting download...
[2017-10-27 @ 17:19:53.829] [reactors/fetch] Fetching the usuals
[2017-10-27 @ 17:20:03.455] [butler] butler: Downloading 16 EiB
[2017-10-27 @ 17:20:03.456] [butler] butler: Resuming at 0 B / 16 EiB
[2017-10-27 @ 17:20:04.637] [butler] butler error: truncate /home/[redacted]/.config/itch/downloads/182904: invalid argument
[2017-10-27 @ 17:20:04.640] [download-watcher] Download threw
[2017-10-27 @ 17:20:04.641] [download-watcher] Download ended, err: truncate /home/[redacted]/.config/itch/downloads/182904: invalid argument
[2017-10-27 @ 17:20:04.642] [reactors/tasks] Download had an error, should notify user
[2017-10-27 @ 17:20:04.876] [download-watcher] Cancelling/clearing out last download
Looks like an itch.io bug, not a game specific bug?