I seemed to have acquired a spurious entry that makes a (non-existent) download/update request. I can't find an entry to delete it.
In looking at my back-up files, the last correct stock-price upload occurred on January 30, 2017.

The symbol in the download/update screen is shown as "USD>E000066", which is a non-existent security.
The ID is "E000066"
The source is identified as "yahoo currency"

I have not been able to locate a screen that would allow me to delete that entry.

How can that entry be deleted from the "upload" request?

Issue solved. Under "tools/prices" I was able to find a matching entry. Repeated deleting of that entry finally resulted in "USD>E000066" disappearing and the price upload working as expected. What happened to create that entry is unknown, but the issue is now solved.