I fixed this yesterday. (Or maybe Monday? I am at a hackathon right now and my sense of time is very diminished ). Back in Madison, our student sysadmin was migrating domains from our old web server to the new one; when
downloads.imagej.net migrated, this problem happened due to a broken symlink. I did a quick hacky fix to get the links working again, but they may become slightly out of date until the Jenkins job which keeps those downloads up-to-date is adjusted to point to the new web server.
Let us know ASAP if the problem recurs.