Thanks for the input, guys. I tried reducing my overlay size to 2048 pixels wide (still no good), then to 1024 pixels, which worked. Unfortunately, at that resolution the hiking trail isn't very visible. I suppose I could create multiple smaller overlays to cover the area.

I'm hopeful, as PenguinOpus suggests, that this is a bug that will be fixed. I'm still curious why it worked the first time, but not on subsequent tries.

Kev