I have just noticed that Spark’s nightly builds list is empty, though there were a number of builds a few weeks ago. I suppose builds are removed on a schedule (@Daryl Herzmann)? Maybe the script can be changed to remove builds based on the number of them, not the age, so there always will be latest builds available and not the empty list, if there are no recent activity. Of course, one can find recent builds on Bamboo, but it is faster and more approachable for users on the Downloads page.
P.S. or maybe this is some error, as it seems only Spark is affected by this and other projects still have their older builds showing.