I checked, all the levels were there. I checked the level properties in the level editor to see which one led where to try to work around the problem, but it didn't help.
It turns out I didn't get a response on the forum that day (I'm not blaming anyone) and replayed some of the levels.
I usually use two or three save slots as backups. Because cases of player characters getting stuck in level structures still occur.
I had a save file when I had already reached Gargoyles Lair, and there was an older save file on the first level of the Tube Electric scene. (Don't remember the right name of the map).
When I got the file not found error, I started playing from the second save, reached Medivo 1, skipped the secret level, and simply completed it and moved on to Medivo 2. I finished the entire game that same day, so I had no further problems. After replaying, I uninstalled and reinstalled the game.
I saw your message a few days later and decided to test it. And everything worked. However, during the test, I simply "jjnext" into Medivo1 using the code, reached the secret, and completed it, returning to Medivo1 to the location of the secret level entrance.
When I first played and discovered the problem, I started from the very beginning of the Secret Files episode, advancing through the game from episode to episode. Who knows? Perhaps the bug was in the save file itself.
But I can try playing through the entire game again, attempting to access this secret level, and if the error persists, I'll take a screenshot.