very odd, i managed to get it working, but the solution made no sense.
i increased the Tile Pool Size (which was already high), and then it worked. then i changed it back to what it was before, and it kept working.
no idea why that might have helped.
i also found that when i had navmeshes in the levels that were generated with Static, and then switched to Dynamic Modifiers Only without regenerating them, the navmeshes were very broken in game. Regenerating them fixes that though.