If you are just getting the no flight available then I think that message does mean the tool can't find a flight. The other messages do mean there is a routing error in my experience. Please post if you find out differently.

Do you want to book using the tool? If so it will have to show flight availability.

If you are just checking routing and possible costs (which is what I use the tool for) then just adjust dates until your routing prices. Then you know the route is valid and the approximate price.
Even if you get a route that prices whoever you book with may show different availability and different price but you should be close enough to get started booking for the dates you want.

Also I find it helpful to save itineraries that do price to fall back to.

And taking a break often magically makes things start working.