Thanks for the file, Anders.
If you use the Extrude option, have the Create Cap option off (Which is my best idea of how this could have happened).
In the Attribute Manager> Mode> Modeling> Mesh Checking.
In this case, the “Non-Manifold” showed four edges, indicating that something in that area was off.
Another indicator was in the Structure Manager> Polygons. An object like that should have an even number of polygons, not an odd number (0-14=15)
Those “internal” polygons are often a source for problems (e.g., SubDivision Surface).
Select one (or more) polygon(s), then use Select> Hide Selected. This allows you to see it. Select and delete that internal polygon, then use Unhide All.
ENJOY.