AK. In this circumstance the sequence* of events is:
1) R-click on the desired file and "Open with SPP6".
2) The browser window begins to open and you get the error message.
3) Click on that and after a bit of disk activity you get the single file icon in the browser window,
and soon after the edit window opens up.
4) If you close that, then the browser window remains open and the Folders panel is populated with ALL sub-folders listed
on the drives it can see, BUT you still DO NOT see C and D.
However, you CAN save a processed file and it will end up in the folder it came from.
* Might as well be sequins FWIW