It fixed itself, here's what I noticed. I was right clicking on the project folder and using a contextual menu to choose New > File, invoking that dialog window. The 'create' button was greyed out. I reconfirmed with another project. But, when I used the main menu File, New (which I had never tried ...

Hi, it was from the project context. I'll email you a screenshot.

When I use the create new file command in my project the wizard shows me a menu of templates. I can double click on one to create an 'untitled.dita' file; however if I use the Title and File Name options to customize the new file the Create Button will not activate. Only the Cancel button is active....