It took me some time, but it works!
Here’s what I think went wrong:
I first downloaded the 1914 Improved version from here:
https://www.mediafire.com/file/6iqf74g0s1zyktp/world_war_i_1914_balanced.zip/file
Which worked well for me for a while.
When I deleted it, I redownloaded that old version which doesn’t work.
Only after looking back through this thread did I notice that 1914 Improved was available in the “Download Maps” section. I deleted the old map and downloaded the new one from there, and it works now.