The technical team of the ADG web site and WHS forums (also known as nwadmin) were helpful in solving my logon problems from my iPhone. Thanks to Matt Costa and the others who searched the databases and tried lots of things.
It turns out that it was something none of us would have guessed. I was logging on using a mixed-case version of my email address. Most systems automatically convert email ids to lower case. For some reason, the ADG database wasn’t matching my mixed case version with the lower case version it had stored. That may have already been fixed. But it explains why the error could not be recreated.
That’s what I get for being too clever with my email address.