Found the problem. it was the plugin nodebb-plugin-merge-subtopics
It could load any category page with sub-categories in it.
Maybe it's hepfull for somebody who has same problem
Is there any way to migrate NodeDB data from one to the other?
Not yet, right now we have import from phpBB, vB, and UBB using this strategy. Theoretically someone could write an exporter from NodeBB to NodeBB using this same strategy.