well, i understand that it's about a region with too many portals but i want to know how i can prevent this.
but i didn't just delete the wrong part, i'll explain:

If you got three parts in a level, 1, 2 and 3.
I first deleted part 1 (and let part 2 and 3 in the level),
then i deleted part 2 (and let part 1 and 3 in the leve),
then i deleted part 3 (with part 1 and 2 in the level). All didn't work.
Finally i deleted part 1, 2 and 3 at the same time. That worked.

And about the portals, there isn't a part in my level that has many more portals than other parts or other levels (the level isn't even detailed), and i don't have very small cubes in my level (most of the blocks are walls)