I had the same problem with an Windows/IE laptop once. The solution wasn't very logical but anyay, it worked.
On the IE there was an option called "Do not save encrypted pages to disk" after changing the settings (can't recall if this was en- or disabled) it worked.
Maybe you want to give this a try.
I know but as a MAC has a browser too, you may want to look if there is a corresponding setting in Safari which solves the problem like it did for my IE issue.