Navigation builddoesn't get completed. What's wrong with it?

Hi all.
I’m working on a top-down game. When I create a maze (at beginplay), the navigation does seem to get built, but doesn’t get completed.

You can see in the picture that at some point it stops building.
I messed around with cell size and agent radius for quite a long time, only to fail to see what the problem is.

Anyone got any solution for this?
Save me, please. I’m kinda desperate.

(FYI, I checked “Force Rebuild on Load” to true and set Runtime Generation to “Dynamic” in the RecastNavMesh. Not sure it is relevant, though.)