You are not doing anything wrong. The server is borked for the latest version, 2.7.2, and has been for some time.
Just tell the Board Manager to install version 1.12.0
It works for me.
Regards,
Dave
Footnote: I think that the hardest job in the world is in IT for servers like this. No one appreciates you, personally, when things work out OK, and every one absolutely hates you, personally, when things go wrong. Sometimes they even accuse of not testing this stuff when you put up new versions.