403 means “iPhone Invalid Authentication”, so probably the username/password is incorrect.
They must be configured in the Keyboard Maestro Web Server preferences. Make sure you tab out of the password field after entering as it seems sometimes if you don’t it wont be set correctly.
Then make sure the username and password is entered correctly in Keyboard Maestro Control.
I would not recommend using the same password as your Mac password, although once the connection is made the password is no longer stored, but even so, a different password would be advisable.
Instructions worked perfectly – KM Control now working on my iPad.
Thank you, @peternlewis
One little detail still worries me:
Why couldn’t I find this myself, without bothering you and cluttering up the forum?
I already have a search set for finding answers about KM.
It finds something about almost everything, but nothing about this.
There is some mistake in my searching, but what?