Thanks, Dick. I confirmed that I have uninstalled OneDrive, then made the registry change suggested in #1.
indeed, Planner now opens my normal Documents folder when I save the plan. But it also gives me the warning about the flat filename being non-portable because it's outside ACP's Plans folder (see screen shot). How does Planner know about ACP's Plans folder?
The #duskflats directive in my plan looks like this:
;WARNING: The following file path/name is not portable.
#duskflats C:\Users\mikey\Documents\ACP Astronomy\Plans\xxx-flats.txt ; Acquire flat fields at dusk
The problem is that full path to my office PC's Documents folder. I need just the filename, no path. Planner used to do that, which is why I'm surprised at this behavior.
--- Mike