That is a fairly common problem/message when you have a route saved that was created from an earlier version of the map.
I don't have a Montana so I don't have the direct solution but basically you have to get it to somehow recalculate using the new map or manually re-create the route with the new map and save it again.
Maybe this will trigger an idea for you....
This happens all the time when updating maps on a Nuvi 7xx when routes have been previously saved. The Nuvi solution is quite simple. I simply delete all my saved routes and re-import them. The Nuvi automatically goes and find them in the appropriate GPX file where they were initially saved and re-imports them one at a time. During the import, it recalculates and re-saves them to current.gpx using the currently selected map. After that, they all work.



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