A week ago I had this issue come back as well. Changing my download server in Steam fixed the problem. I've updated and I'm still problem free.

After a Steam update today I cannot repeat the issue. All of the previous scenarios downloaded as I went to reproduce. I can still send the RPT from my attempt but it seems to be fixed now. Again, I didn't notice an Arma update but Steam updated just before I restarted it so I could start it with the logs parameter.

I am experiencing this same issue but it gets stuck at 40%. I am not running the dev build.