Okay, I just tested this. And I can confirm the problem.
I did notice this morning, that there were some updates available for 15.1. But they have not updated the install image on the download page. My best guess is that this is the cause of the problem. The NET installer actually downloads the install from the online repos. And apparently it is noticing that what it can download does not match what it expects, probably because the only repos have been updated more recently than the NET installer.
My suggestions:
(1) wait for the next NET installer version;
(2) in the meantime, file a bug report so the that 15.1 team are made aware that what they did does not work.