I edited the interior cell "MarkarthVlindrelHall" and added a second door leading to a terrace in the cell "MarkarthOrigin" in Markarth World.
When linking the doors together via teleport dialog I get an array of error messages like that:

CELLS: Multibound for ref xxxxxxxx in cell MarkarthVlindrelHall (00016DFA) is outside the loaded world's extents

xxxxxxxx stands for the varying ref id numbers. The refs concerned are only roommarkers. 6 refs are concerned, after the first 6 messages with their respective ref id's the array repeats to a total of 12 messages, means 2 identicel messages for each of the ref id's. They are spread all over the cell (MarkarthVlindrelHall) and there is nothing they seem to have in common - at least nothing obvious to me.

Nevertheless the CK allows me to connect he doors, place the markers accordingly and also the navmesh finalises seemingly correct (by displaying the green triangle).
But ingame I get a CTD without message when trying to use the respective doors, no matter if trying to go "outside" or "inside".
Apart from these issues I get no other error messages and all portals work correctly.

Any hints would be greatly appreciated.

PS:
Just to cover every aspect - here is the way I use to link the doors:

1. I place one instance of "dewdooesmall01load" in both cells (MarkarthVlindrelHall and MarkarthOrigin).
2. I edit the door in Vlindrel Hall and check the "teleport" box.
3. With the open edit window I open MarkarthOrigin cell
4. I click on "Select reference in Render Window" and double click on the door in MarkarthOrigin cell
5. I click "OK" button
6. This is the moment the error messages pop up. It does not matter if I exit the array or confirm them one after another. In both cases I end up with teleport markers and am able to finish the process. In both cases the teleport is not working as described above.

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Edit:

I deleted all rooms and portals.
I got rid off the error messages when linking the doors - but the CTD persists.

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Edit 2:

Has turned out to be a major problem. I also get CTD when usinf the main door. That means no exiting the cell "MarkarthVlindrelHall" possible at all.