Same for me, the Navmesh is broken, using a unmodded save and reinstalled Skyrim did'nt help. Seems like I have to do 2500 triangles again. Also tried to refinalize the interior and removed a part of the navmesh but it did'nt help the cell only loads when the navmesh is removed completely. Created some new triangles after removing the old navmesh and finalized to test, everything is fine.