We've tried downloading a FIT file for that route with the app on an Android device and it worked OK for us. You should see something like this:

Do you have possibly any apps or browser plug-ins installed that could be blocking the download? It would be worth clearing the app's cache (on your My Settings page in the plotaroute app) to see if this helps or uninstalling the app and reinstalling it. I'm not sure what else to suggest.
I'm using an android phone.
I've tried a number of does without success, and have not found one that works. Here is one that fails too download from the app.
https://www.plotaroute.com/route/1495950?units=miles
I tried to download by selecting the fit icon on the route info page.
It's working OK for us here Michael. Can you share further details as it may relate to specific circumstances? In particular, what device you're using (iPhone/Android), what route you're downloading and what download settings you've chosen.
I get a 404 error when I try to download a route with the app. I had success using the mobile website.