I had this problem with Norton on one of my computers. I had to disable a feature called “Download Intelligence” which was deciding the downloads were dangerous because not enough Norton customers had downloaded those installers.
I could not find any sign that the downloads had been quarantined. I could not find any way to get them back. I first tried to disable “Download Intelligence” for a day, but when the day was up, Norton woke up and deleted all the files I had downloaded while the feature was disabled. Permanently disabling the feature seems to be the only option.