After exporting a Trello board as a JSON file, is it possible to import it again? (in the same/new account)
If you exported the JSON with the intention of re-importing the board elsewhere, you should transfer the board instead.
If you want to copy the board to a new team:
- Copy the board you want in the origin team (Menu > More... > Copy Board)
- Transfer the board copy to the destination team
If you exported a JSON and you don't have access anymore to the original board, then you should follow boardtc's advice.