NavMesh Bug

There is an issue with Skyrim where actors sometimes cannot navigate in cells that have been created by mods or in vanilla Skyrim cells that have had their NavMeshes edited by mods.

The actors will be frozen to a spot. They can't do their sandbox animations because they can't get to the markers. Enemies may not be able to attack the player character because they cannot get to melee weapon range. Followers will stop following the player character.

The NavMeshes only fail to be followed by actors when they are in .esp files. The NavMeshes are always followed properly by actors when the data is in an .esm. (Like Skyrim.esm for example.)

The Solution

The current work-around for this issue is to place NavMesh data from a new mod into an .esm. This could take the form of having a mod be an .esm with no .esp, but a better approach would often be to make a mod into an .esp/.esm pair where the .esp uses the .esm as a master. The NavMesh data is the only thing that absolutely must be in the .esm. The rest of the content of the mod could be in the .esp which is easier to edit.