I just ran into this same problem today. Usually when I have these kinds of problems it's because of my K9 Web Protection. I tried telling K9 to always allow minecraft.net...didn't work. Tried to always allow assets.minecraft.net...didn't work. However, if you go to assets.minecraft.net by typing it into your address bar...it will take you to a page where at the top of the page it shows a website called s3.amazonaws.com. So I told K9 to always allow s3.amazonaws.com and it worked! Even if you don't have K9, I suggest that you tell whatever antivirus program you have to always allow s3.amazonaws.com. Also keep in mind, your antivirus may have never blocked minecraft before but like cestislife said these programs tend to update themselves which means your antivirus may very well block minecraft now. I hope this helps.
I had K9, and I followed these steps, and it unblocked it for me. Thanks!
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I had K9, and I followed these steps, and it unblocked it for me. Thanks!