Thanks for the suggestion, but it did not work. In the interest of adding to knowledge, I will describe what I seem to have done, and how it was repaired. In the program, there are a number of child forms that are activated when I click a check box, allowing selecting one of a list of options to fill said check box. I must have removed these forms as i was editing the program. As I tried to work my way through, after removing and reinstalling Visual Studio, with the program running, I happened to try a modify on one of said child forms. Up came an error message, can't find this form. I selected locate, and lo! the form was in the full database but was not in the'project database' and so was not a part of the working program. Indicated the location, and all worked. Why removing such child forms gave the 'APP_MEDIATOR' error I have no idea, but as given here such are the facts of my problem and correction of same.