I had exactly the same problem, with sccm 2012 SP1 on a server 2012R2
I simply enabled anonymous:
Administration -> Site Configuration ->
Select Servers and site system roles
Double-click on the Distribution point
General tab: check the "allow clients to connect anonymously" box then click on OK
FYI, this only helps if you are not running "HTTPS Only" mode. In "HTTPS Only", the option you mention is grayed out.
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