Thanks for your response, blinkOK!
I've figured out what caused this and thought it worth mentioning because I had to compare the AppGameKit files of the broken and working versions to find the cause. The broken Project.AGK file contained this line:
watch_variable=[g_intValidMoves.Length;
Note the unbalanced opening square bracket? AppGameKit added this line to the project file when I asked to check the value of "[g_intValidMoves.Length", fat-fingering the bracket while typing, and then closed the project. AppGameKit won't let us check properties in the debugger anyway, but made no mention of square bracket being an invalid character in the watch list and undoubtedly saved it to the project file for my "convenience" the next time I opened the project. Mea culpa...but the interface ought to warn and/or refuse to save invalid characters in the project file.