I only managed to get the prison working for version 1.18, which does not contain all scenes. Missing the DLC mobs such as goblins, majin boss, and succubus, as well as the cursed gear scene.
You need to go to the final boss Medusa and lose against her. The entrance can be found on the lower right corner of floor 7 in the cave dungeon. After losing the game will give instructions to hold "V" while loading saves which should have a star symbol next to them.
Holding "V" on a clear game save provided by other people did not work, so i assume there's a registry or AppData value being written somewhere. Losing to Medusa in version 2.02 is not possible if you are using someone else's save, as she bugs out and just stands there taking 0 damage.
According to DLsite there's an update to the DLC for monsters taking zero damage. Until a download for the fix becomes available the only way to unlock the prison would be to clear the game yourself.
Update information
- Oct/12/2024 Bug fixes ・モンスター情報の出る一部の魔物が初回以降無敵になるバグを修正しました。