Eureka! Plan-G does support OpenStreetMap!

Go to File > Options > General and select the OpenStreetMap radio button.

Ta da! All is good.  :)

Looking at the MapQuest plans my guess is that it will no longer be possible for Mr. Arnot to offer Plan-G by donation if he continues to use MapQuest. So it looks like it is now necessary for Plan-G to use OpenStreetMap (as it says on the home page) instead of MapQuest.

It's better than MapQuest anyways. Here's the standard layer (it can be changed to include terrain): https://www.openstreetmap.org/relation/65606#map=9/51.7245/-0.7237

This is all over the Internet today for every application that uses MapQuest. Here is an example:

http://stackoverflow.com/questions/38315300/mapquest-direct-tile-access-discontinued

So MapQuest has screwed a lot of people. This could take a while to fix.