This was simply due to my ignorance. The category was visible but the only related package was not. After selecting the category I was expecting the next step to be package selection which of course couldn't happen because there was no visible package in that category. Doh!
I'm having the same issue. The error log reported that the clientarea.php file did not exist in the admin directory so I copied it there (why not). That stopped the error but the add order page still just waits and waits...
Any thoughts?