I'd been having issues with this myself recently. I assume you're in Godot 4.1? I tracked the issue down to being cyclic references in GDScript. As an example from what you've provided, you may have LevelBase referring to the Player script, and then the Player script referring to LevelBase. This only seems to be a problem with the type hinting in GDScript like the following:
var player:PlayerClass
Removing the hinting may fix it. As for the bug, you can read some more on it here: https://github.com/godotengine/godot/issues/80877
As a side note, I've noticed that 4.1 has this issue worse than 4.0. In 4.0 it just unbinds the script from the scene, and allows you to rebind it again. But in 4.1 it causes the error message, making you unable to open the scene.
Seems to be comparing the waterfalls.