Hello,
It is impossible from v6.21 to get any ISO files from Build v17744 family, whatever it is (ADK, WinPE, Client Insider).
Error 404 or No response to the request are offered from several days.
Is it a Browser (64bit) issue ? I do tests with Firefox v62, or Vivaldi v1.16 or Palemoon v28.0.1 or Cyberfox v52.9.1.
Formerly, any Build v17738 family ISO files was downloadable (with v6.20), now it does not work using v6.21.
So, it does seem a severe SERVER problem somewhere.