In order of what it usually is:
1. Did you originally clone the cell from a vanilla cell? If you did, finalise the navmesh. If that doesn't work, sadly, delete your navmesh and start it again.
2. Can a follower enter the interior cell? If not, finalise the navmesh (assuming you have navmeshed it!?)
3. Are the green/blue triangles of the Yellow Door Markers sitting nicely on the navmesh? If not, shift them about a bit so they are nice and square on the grid.
4. Does your interior cell have lots and lots and lots of clutter? If it does (and I do mean lots and lots and lots and lots) then there's a chance you are seeing the memory issue. To fix that either remove some clutter, OR split your interior across multiple cells
It's usually one of that list that's causing this problem ... best of luck tracking it down
