Is the authad-plugin activated in your configuration manager?
It is, yes.
Make sure it's enabled in the Extension Manager. If for some reason it has been deleted, make sure you have the latest dokuwiki, and then download a separate copy of dokuwiki and in lib/plugins you will find authad. Copy its whole directory to lib/plugins and make sure it's read-write to the web server. To get the latest dw, if you don't have it, use the upgrade plugin; consult the dw wiki for upgrade instructions.
That got it back - thanks very much for your help, Myron.
I still can't get it to work though - as soon as I activate it I get an error
Something unforseen has happened: Maximum action transitions reached
...which is less than informative.
I guess I have more reading to do.
Anyhoo, thanks for taking the time to help.